diff --git a/PROJECTS/intermediate/credential-rotation-enforcer/spec/unit/audit/audit_log_spec.cr b/PROJECTS/intermediate/credential-rotation-enforcer/spec/unit/audit/audit_log_spec.cr
new file mode 100644
index 00000000..690c08e9
--- /dev/null
+++ b/PROJECTS/intermediate/credential-rotation-enforcer/spec/unit/audit/audit_log_spec.cr
@@ -0,0 +1,62 @@
+# ===================
+# ©AngelaMos | 2026
+# audit_log_spec.cr
+# ===================
+
+require "../../spec_helper"
+require "../../../src/cre/audit/audit_log"
+require "../../../src/cre/persistence/sqlite/sqlite_persistence"
+
+describe CRE::Audit::AuditLog do
+  it "appends and reads back entries with valid hash chain" do
+    persist = CRE::Persistence::Sqlite::SqlitePersistence.new(":memory:")
+    persist.migrate!
+    log = CRE::Audit::AuditLog.new(persist, Bytes.new(32, 0_u8), 1, 1024)
+
+    log.append("rotation.completed", "system", nil, {"x" => "y"})
+    log.append("policy.violation", "system", nil, {"a" => "b"})
+
+    persist.audit.latest_seq.should eq 2_i64
+    log.verify_chain.should be_true
+  ensure
+    persist.try(&.close)
+  end
+
+  it "verify_chain detects when a row is tampered in DB directly" do
+    persist = CRE::Persistence::Sqlite::SqlitePersistence.new(":memory:")
+    persist.migrate!
+    log = CRE::Audit::AuditLog.new(persist, Bytes.new(32, 0_u8), 1, 1024)
+
+    log.append("a", "s", nil, {"k" => "v"})
+    log.append("b", "s", nil, {"k" => "v2"})
+
+    persist.db.exec("UPDATE audit_events SET payload = ? WHERE seq = 2", %({"event_type":"b","actor":"s","target_id":null,"payload":{"k":"BAD"}}))
+
+    log.verify_chain.should be_false
+  ensure
+    persist.try(&.close)
+  end
+
+  it "verify_chain returns true on empty log" do
+    persist = CRE::Persistence::Sqlite::SqlitePersistence.new(":memory:")
+    persist.migrate!
+    log = CRE::Audit::AuditLog.new(persist, Bytes.new(32, 0_u8), 1, 1024)
+    log.verify_chain.should be_true
+  ensure
+    persist.try(&.close)
+  end
+
+  it "ratchets version after configured threshold" do
+    persist = CRE::Persistence::Sqlite::SqlitePersistence.new(":memory:")
+    persist.migrate!
+    log = CRE::Audit::AuditLog.new(persist, Bytes.new(32, 0_u8), 1, ratchet_every: 2)
+
+    log.append("a", "s", nil, {"i" => "1"})
+    log.append("a", "s", nil, {"i" => "2"})
+    log.ratchet_version.should eq 1
+    log.append("a", "s", nil, {"i" => "3"})
+    log.ratchet_version.should eq 2
+  ensure
+    persist.try(&.close)
+  end
+end
diff --git a/PROJECTS/intermediate/credential-rotation-enforcer/spec/unit/audit/batch_sealer_spec.cr b/PROJECTS/intermediate/credential-rotation-enforcer/spec/unit/audit/batch_sealer_spec.cr
new file mode 100644
index 00000000..d1f1e2e0
--- /dev/null
+++ b/PROJECTS/intermediate/credential-rotation-enforcer/spec/unit/audit/batch_sealer_spec.cr
@@ -0,0 +1,73 @@
+# ===================
+# ©AngelaMos | 2026
+# batch_sealer_spec.cr
+# ===================
+
+require "../../spec_helper"
+require "../../../src/cre/audit/audit_log"
+require "../../../src/cre/audit/batch_sealer"
+require "../../../src/cre/audit/signing"
+require "../../../src/cre/persistence/sqlite/sqlite_persistence"
+
+describe CRE::Audit::BatchSealer do
+  it "seals pending audit events into a signed Merkle batch" do
+    persist = CRE::Persistence::Sqlite::SqlitePersistence.new(":memory:")
+    persist.migrate!
+
+    log = CRE::Audit::AuditLog.new(persist, Bytes.new(32, 0_u8), 1, 1024)
+    log.append("a", "s", nil, {"i" => "1"})
+    log.append("b", "s", nil, {"i" => "2"})
+    log.append("c", "s", nil, {"i" => "3"})
+
+    kp = CRE::Audit::Signing::Ed25519Keypair.generate(version: 1)
+    signer = CRE::Audit::Signing::Ed25519Signer.from_keypair(kp)
+    sealer = CRE::Audit::BatchSealer.new(persist, signer)
+
+    batch = sealer.seal_pending.not_nil!
+    batch.start_seq.should eq 1_i64
+    batch.end_seq.should eq 3_i64
+    batch.signing_key_version.should eq 1
+    batch.merkle_root.size.should eq 32
+    batch.signature.size.should eq 64
+
+    verifier = CRE::Audit::Signing::Ed25519Verifier.new(kp.public_key)
+    msg = CRE::Audit::BatchSealer.pack_message(batch.start_seq, batch.end_seq, batch.merkle_root)
+    verifier.verify(msg, batch.signature).should be_true
+
+    persist.audit.last_sealed_seq.should eq 3_i64
+  ensure
+    persist.try(&.close)
+  end
+
+  it "returns nil when nothing pending" do
+    persist = CRE::Persistence::Sqlite::SqlitePersistence.new(":memory:")
+    persist.migrate!
+
+    kp = CRE::Audit::Signing::Ed25519Keypair.generate
+    signer = CRE::Audit::Signing::Ed25519Signer.from_keypair(kp)
+    sealer = CRE::Audit::BatchSealer.new(persist, signer)
+    sealer.seal_pending.should be_nil
+  ensure
+    persist.try(&.close)
+  end
+
+  it "subsequent seal covers only new events" do
+    persist = CRE::Persistence::Sqlite::SqlitePersistence.new(":memory:")
+    persist.migrate!
+    log = CRE::Audit::AuditLog.new(persist, Bytes.new(32, 0_u8), 1, 1024)
+    kp = CRE::Audit::Signing::Ed25519Keypair.generate
+    signer = CRE::Audit::Signing::Ed25519Signer.from_keypair(kp)
+    sealer = CRE::Audit::BatchSealer.new(persist, signer)
+
+    log.append("a", "s", nil, {"i" => "1"})
+    log.append("b", "s", nil, {"i" => "2"})
+    sealer.seal_pending.not_nil!.end_seq.should eq 2_i64
+
+    log.append("c", "s", nil, {"i" => "3"})
+    second = sealer.seal_pending.not_nil!
+    second.start_seq.should eq 3_i64
+    second.end_seq.should eq 3_i64
+  ensure
+    persist.try(&.close)
+  end
+end
diff --git a/PROJECTS/intermediate/credential-rotation-enforcer/spec/unit/audit/hash_chain_spec.cr b/PROJECTS/intermediate/credential-rotation-enforcer/spec/unit/audit/hash_chain_spec.cr
new file mode 100644
index 00000000..278a087f
--- /dev/null
+++ b/PROJECTS/intermediate/credential-rotation-enforcer/spec/unit/audit/hash_chain_spec.cr
@@ -0,0 +1,63 @@
+# ===================
+# ©AngelaMos | 2026
+# hash_chain_spec.cr
+# ===================
+
+require "../../spec_helper"
+require "../../../src/cre/audit/hash_chain"
+
+describe CRE::Audit::HashChain do
+  it "first hash is 32 zero bytes" do
+    g = CRE::Audit::HashChain.genesis
+    g.size.should eq 32
+    g.all? { |b| b == 0_u8 }.should be_true
+  end
+
+  it "next_hash is deterministic" do
+    prev = CRE::Audit::HashChain.genesis
+    payload = "hello".to_slice
+    a = CRE::Audit::HashChain.next_hash(prev, payload)
+    b = CRE::Audit::HashChain.next_hash(prev, payload)
+    a.should eq b
+    a.size.should eq 32
+  end
+
+  it "different prev produces different hash" do
+    payload = "x".to_slice
+    a = CRE::Audit::HashChain.next_hash(Bytes.new(32, 0_u8), payload)
+    b = CRE::Audit::HashChain.next_hash(Bytes.new(32, 1_u8), payload)
+    a.should_not eq b
+  end
+
+  it "different payload produces different hash" do
+    prev = CRE::Audit::HashChain.genesis
+    a = CRE::Audit::HashChain.next_hash(prev, "a".to_slice)
+    b = CRE::Audit::HashChain.next_hash(prev, "b".to_slice)
+    a.should_not eq b
+  end
+
+  it "verify_chain detects tampering" do
+    pairs = [] of {Bytes, Bytes}
+    h = CRE::Audit::HashChain.genesis
+    payloads = ["a", "b", "c", "d"].map(&.to_slice)
+    payloads.each do |p|
+      next_h = CRE::Audit::HashChain.next_hash(h, p)
+      pairs << {h, next_h}
+      h = next_h
+    end
+
+    CRE::Audit::HashChain.verify(pairs, payloads).should be_true
+
+    tampered = payloads.dup
+    tampered[2] = "BAD".to_slice
+    CRE::Audit::HashChain.verify(pairs, tampered).should be_false
+  end
+
+  it "verify_chain returns true for empty input" do
+    CRE::Audit::HashChain.verify([] of {Bytes, Bytes}, [] of Bytes).should be_true
+  end
+
+  it "verify_chain returns false on mismatched array sizes" do
+    CRE::Audit::HashChain.verify([] of {Bytes, Bytes}, ["x".to_slice]).should be_false
+  end
+end
diff --git a/PROJECTS/intermediate/credential-rotation-enforcer/spec/unit/audit/hmac_ratchet_spec.cr b/PROJECTS/intermediate/credential-rotation-enforcer/spec/unit/audit/hmac_ratchet_spec.cr
new file mode 100644
index 00000000..8cfd0195
--- /dev/null
+++ b/PROJECTS/intermediate/credential-rotation-enforcer/spec/unit/audit/hmac_ratchet_spec.cr
@@ -0,0 +1,43 @@
+# ===================
+# ©AngelaMos | 2026
+# hmac_ratchet_spec.cr
+# ===================
+
+require "../../spec_helper"
+require "../../../src/cre/audit/hmac_ratchet"
+
+describe CRE::Audit::HmacRatchet do
+  it "produces 32-byte HMAC" do
+    r = CRE::Audit::HmacRatchet.new(Bytes.new(32, 0_u8), version: 1, ratchet_every: 1024)
+    h = r.sign("payload".to_slice)
+    h.size.should eq 32
+    r.version.should eq 1
+  end
+
+  it "rotates after N entries" do
+    r = CRE::Audit::HmacRatchet.new(Bytes.new(32, 0_u8), version: 1, ratchet_every: 3)
+    3.times { r.sign("x".to_slice) }
+    r.version.should eq 1 # not yet rotated
+    r.sign("x".to_slice)
+    r.version.should eq 2 # rotated on the 4th call
+  end
+
+  it "different keys produce different HMACs" do
+    a = CRE::Audit::HmacRatchet.new(Bytes.new(32, 0_u8), 1, 1024).sign("x".to_slice)
+    b = CRE::Audit::HmacRatchet.new(Bytes.new(32, 1_u8), 1, 1024).sign("x".to_slice)
+    a.should_not eq b
+  end
+
+  it "rejects keys of incorrect size" do
+    expect_raises(ArgumentError) do
+      CRE::Audit::HmacRatchet.new(Bytes.new(16), 1, 1024)
+    end
+  end
+
+  it "verify (static) round-trips" do
+    key = Bytes.new(32, 0xff_u8)
+    payload = "p".to_slice
+    h = OpenSSL::HMAC.digest(:sha256, key, payload)
+    CRE::Audit::HmacRatchet.verify(payload, h, key).should be_true
+  end
+end
diff --git a/PROJECTS/intermediate/credential-rotation-enforcer/spec/unit/audit/merkle_spec.cr b/PROJECTS/intermediate/credential-rotation-enforcer/spec/unit/audit/merkle_spec.cr
new file mode 100644
index 00000000..4cd7dcce
--- /dev/null
+++ b/PROJECTS/intermediate/credential-rotation-enforcer/spec/unit/audit/merkle_spec.cr
@@ -0,0 +1,38 @@
+# ===================
+# ©AngelaMos | 2026
+# merkle_spec.cr
+# ===================
+
+require "../../spec_helper"
+require "../../../src/cre/audit/merkle"
+
+describe CRE::Audit::Merkle do
+  it "single leaf root equals the leaf" do
+    leaf = "x".to_slice
+    CRE::Audit::Merkle.root([leaf]).should eq leaf
+  end
+
+  it "balanced tree root is deterministic" do
+    leaves = ["a", "b", "c", "d"].map(&.to_slice)
+    r1 = CRE::Audit::Merkle.root(leaves)
+    r2 = CRE::Audit::Merkle.root(leaves)
+    r1.should eq r2
+    r1.size.should eq 32
+  end
+
+  it "different leaves produce different roots" do
+    a = CRE::Audit::Merkle.root(["a", "b"].map(&.to_slice))
+    b = CRE::Audit::Merkle.root(["a", "c"].map(&.to_slice))
+    a.should_not eq b
+  end
+
+  it "odd leaf count is supported (last is promoted)" do
+    leaves = ["a", "b", "c"].map(&.to_slice)
+    r = CRE::Audit::Merkle.root(leaves)
+    r.size.should eq 32
+  end
+
+  it "raises on empty input" do
+    expect_raises(ArgumentError) { CRE::Audit::Merkle.root([] of Bytes) }
+  end
+end
diff --git a/PROJECTS/intermediate/credential-rotation-enforcer/spec/unit/audit/signing_spec.cr b/PROJECTS/intermediate/credential-rotation-enforcer/spec/unit/audit/signing_spec.cr
new file mode 100644
index 00000000..e3292c9d
--- /dev/null
+++ b/PROJECTS/intermediate/credential-rotation-enforcer/spec/unit/audit/signing_spec.cr
@@ -0,0 +1,55 @@
+# ===================
+# ©AngelaMos | 2026
+# signing_spec.cr
+# ===================
+
+require "../../spec_helper"
+require "../../../src/cre/audit/signing"
+
+describe CRE::Audit::Signing do
+  it "generates a keypair" do
+    kp = CRE::Audit::Signing::Ed25519Keypair.generate(version: 1)
+    kp.private_key.size.should eq 32
+    kp.public_key.size.should eq 32
+    kp.version.should eq 1
+  end
+
+  it "signs and verifies a message" do
+    kp = CRE::Audit::Signing::Ed25519Keypair.generate
+    signer = CRE::Audit::Signing::Ed25519Signer.from_keypair(kp)
+    verifier = CRE::Audit::Signing::Ed25519Verifier.new(kp.public_key)
+
+    msg = "audit batch root".to_slice
+    sig = signer.sign(msg)
+    sig.size.should eq 64
+    verifier.verify(msg, sig).should be_true
+  end
+
+  it "rejects tampered message" do
+    kp = CRE::Audit::Signing::Ed25519Keypair.generate
+    signer = CRE::Audit::Signing::Ed25519Signer.from_keypair(kp)
+    verifier = CRE::Audit::Signing::Ed25519Verifier.new(kp.public_key)
+
+    msg = "original".to_slice
+    sig = signer.sign(msg)
+    verifier.verify("tampered".to_slice, sig).should be_false
+  end
+
+  it "rejects tampered signature" do
+    kp = CRE::Audit::Signing::Ed25519Keypair.generate
+    signer = CRE::Audit::Signing::Ed25519Signer.from_keypair(kp)
+    verifier = CRE::Audit::Signing::Ed25519Verifier.new(kp.public_key)
+
+    msg = "x".to_slice
+    sig = signer.sign(msg)
+    sig[0] ^= 0x01_u8
+    verifier.verify(msg, sig).should be_false
+  end
+
+  it "two keypairs produce different keys" do
+    a = CRE::Audit::Signing::Ed25519Keypair.generate
+    b = CRE::Audit::Signing::Ed25519Keypair.generate
+    a.private_key.should_not eq b.private_key
+    a.public_key.should_not eq b.public_key
+  end
+end
diff --git a/PROJECTS/intermediate/credential-rotation-enforcer/src/cre/audit/audit_log.cr b/PROJECTS/intermediate/credential-rotation-enforcer/src/cre/audit/audit_log.cr
new file mode 100644
index 00000000..bbca72b3
--- /dev/null
+++ b/PROJECTS/intermediate/credential-rotation-enforcer/src/cre/audit/audit_log.cr
@@ -0,0 +1,71 @@
+# ===================
+# ©AngelaMos | 2026
+# audit_log.cr
+# ===================
+
+require "json"
+require "uuid"
+require "./hash_chain"
+require "./hmac_ratchet"
+require "../persistence/persistence"
+require "../persistence/repos"
+
+module CRE::Audit
+  class AuditLog
+    @ratchet : HmacRatchet
+    @mutex : Mutex
+
+    def initialize(@persistence : Persistence::Persistence, initial_hmac_key : Bytes, @hmac_version : Int32, @ratchet_every : Int32)
+      @ratchet = HmacRatchet.new(initial_hmac_key, @hmac_version, @ratchet_every)
+      @mutex = Mutex.new
+    end
+
+    def append(event_type : String, actor : String, target_id : UUID?, payload : Hash) : Persistence::AuditEntry
+      @mutex.synchronize do
+        prev = @persistence.audit.latest_hash
+        canonical = canonical_json(event_type, actor, target_id, payload)
+        content_hash = HashChain.next_hash(prev, canonical.to_slice)
+        hmac = @ratchet.sign(content_hash)
+
+        entry = Persistence::AuditEntry.new(
+          seq: 0_i64,
+          event_id: UUID.random,
+          occurred_at: Time.utc,
+          event_type: event_type,
+          actor: actor,
+          target_id: target_id,
+          payload: canonical,
+          prev_hash: prev,
+          content_hash: content_hash,
+          hmac: hmac,
+          hmac_key_version: @ratchet.version,
+        )
+        @persistence.audit.append(entry)
+        entry
+      end
+    end
+
+    def verify_chain : Bool
+      latest = @persistence.audit.latest_seq
+      return true if latest == 0
+      entries = @persistence.audit.range(1_i64, latest)
+      return false if entries.size != latest
+      pairs = entries.map { |e| {e.prev_hash, e.content_hash} }
+      payloads = entries.map(&.payload).map(&.to_slice)
+      HashChain.verify(pairs, payloads)
+    end
+
+    def ratchet_version : Int32
+      @ratchet.version
+    end
+
+    private def canonical_json(event_type, actor, target_id, payload) : String
+      {
+        event_type: event_type,
+        actor:      actor,
+        target_id:  target_id.try(&.to_s),
+        payload:    payload,
+      }.to_json
+    end
+  end
+end
diff --git a/PROJECTS/intermediate/credential-rotation-enforcer/src/cre/audit/batch_sealer.cr b/PROJECTS/intermediate/credential-rotation-enforcer/src/cre/audit/batch_sealer.cr
new file mode 100644
index 00000000..58f1ebac
--- /dev/null
+++ b/PROJECTS/intermediate/credential-rotation-enforcer/src/cre/audit/batch_sealer.cr
@@ -0,0 +1,61 @@
+# ===================
+# ©AngelaMos | 2026
+# batch_sealer.cr
+# ===================
+
+require "uuid"
+require "./merkle"
+require "./signing"
+require "../persistence/persistence"
+require "../persistence/repos"
+
+module CRE::Audit
+  class BatchSealer
+    def initialize(
+      @persistence : Persistence::Persistence,
+      @signer : Signing::Ed25519Signer,
+    )
+    end
+
+    def seal_pending : Persistence::AuditBatch?
+      latest = @persistence.audit.latest_seq
+      last_end = @persistence.audit.last_sealed_seq
+      return nil if latest <= last_end
+
+      start_seq = last_end + 1
+      end_seq = latest
+      entries = @persistence.audit.range(start_seq, end_seq)
+      return nil if entries.empty?
+
+      leaves = entries.map(&.content_hash)
+      root = Merkle.root(leaves)
+
+      msg = pack_message(start_seq, end_seq, root)
+      sig = @signer.sign(msg)
+
+      batch = Persistence::AuditBatch.new(
+        id: UUID.random,
+        start_seq: start_seq,
+        end_seq: end_seq,
+        merkle_root: root,
+        signature: sig,
+        signing_key_version: @signer.version,
+        sealed_at: Time.utc,
+      )
+      @persistence.audit.insert_batch(batch)
+      batch
+    end
+
+    def self.pack_message(start_seq : Int64, end_seq : Int64, root : Bytes) : Bytes
+      io = IO::Memory.new
+      io.write_bytes(start_seq, IO::ByteFormat::BigEndian)
+      io.write_bytes(end_seq, IO::ByteFormat::BigEndian)
+      io.write(root)
+      io.to_slice
+    end
+
+    private def pack_message(start_seq : Int64, end_seq : Int64, root : Bytes) : Bytes
+      BatchSealer.pack_message(start_seq, end_seq, root)
+    end
+  end
+end
diff --git a/PROJECTS/intermediate/credential-rotation-enforcer/src/cre/audit/hash_chain.cr b/PROJECTS/intermediate/credential-rotation-enforcer/src/cre/audit/hash_chain.cr
new file mode 100644
index 00000000..3d8f1948
--- /dev/null
+++ b/PROJECTS/intermediate/credential-rotation-enforcer/src/cre/audit/hash_chain.cr
@@ -0,0 +1,34 @@
+# ===================
+# ©AngelaMos | 2026
+# hash_chain.cr
+# ===================
+
+require "openssl/digest"
+require "../crypto/random"
+
+module CRE::Audit
+  module HashChain
+    GENESIS_SIZE = 32
+
+    def self.genesis : Bytes
+      Bytes.new(GENESIS_SIZE, 0_u8)
+    end
+
+    def self.next_hash(prev_hash : Bytes, payload : Bytes) : Bytes
+      d = OpenSSL::Digest.new("SHA256")
+      d.update(prev_hash)
+      d.update(payload)
+      d.final
+    end
+
+    def self.verify(pairs : Array({Bytes, Bytes}), payloads : Array(Bytes)) : Bool
+      return false unless pairs.size == payloads.size
+      pairs.each_with_index do |entry, i|
+        prev, current = entry
+        expected = next_hash(prev, payloads[i])
+        return false unless CRE::Crypto::Random.constant_time_equal?(expected, current)
+      end
+      true
+    end
+  end
+end
diff --git a/PROJECTS/intermediate/credential-rotation-enforcer/src/cre/audit/hmac_ratchet.cr b/PROJECTS/intermediate/credential-rotation-enforcer/src/cre/audit/hmac_ratchet.cr
new file mode 100644
index 00000000..2dd5d0ce
--- /dev/null
+++ b/PROJECTS/intermediate/credential-rotation-enforcer/src/cre/audit/hmac_ratchet.cr
@@ -0,0 +1,52 @@
+# ===================
+# ©AngelaMos | 2026
+# hmac_ratchet.cr
+# ===================
+
+require "openssl/hmac"
+require "openssl/digest"
+
+module CRE::Audit
+  class HmacRatchet
+    getter version : Int32
+
+    @key : Bytes
+    @counter : Int32
+
+    def initialize(initial_key : Bytes, @version : Int32, @ratchet_every : Int32)
+      raise ArgumentError.new("key must be 32 bytes") unless initial_key.size == 32
+      @key = initial_key.dup
+      @counter = 0
+    end
+
+    def sign(payload : Bytes) : Bytes
+      maybe_rotate
+      h = OpenSSL::HMAC.digest(:sha256, @key, payload)
+      @counter += 1
+      h
+    end
+
+    def self.verify(payload : Bytes, expected : Bytes, key : Bytes) : Bool
+      h = OpenSSL::HMAC.digest(:sha256, key, payload)
+      CRE::Crypto::Random.constant_time_equal?(h, expected)
+    end
+
+    def current_key : Bytes
+      @key.dup
+    end
+
+    private def maybe_rotate : Nil
+      return unless @counter >= @ratchet_every
+
+      d = OpenSSL::Digest.new("SHA256")
+      d.update(@key)
+      d.update("ratchet-v#{@version + 1}".to_slice)
+      new_key = d.final
+
+      @key.size.times { |i| @key[i] = 0_u8 }
+      @key = new_key
+      @version += 1
+      @counter = 0
+    end
+  end
+end
diff --git a/PROJECTS/intermediate/credential-rotation-enforcer/src/cre/audit/merkle.cr b/PROJECTS/intermediate/credential-rotation-enforcer/src/cre/audit/merkle.cr
new file mode 100644
index 00000000..5b177b7d
--- /dev/null
+++ b/PROJECTS/intermediate/credential-rotation-enforcer/src/cre/audit/merkle.cr
@@ -0,0 +1,36 @@
+# ===================
+# ©AngelaMos | 2026
+# merkle.cr
+# ===================
+
+require "openssl/digest"
+
+module CRE::Audit
+  module Merkle
+    def self.root(leaves : Array(Bytes)) : Bytes
+      raise ArgumentError.new("empty merkle tree") if leaves.empty?
+      level = leaves.dup
+      while level.size > 1
+        next_level = [] of Bytes
+        i = 0
+        while i < level.size
+          if i + 1 < level.size
+            next_level << combine(level[i], level[i + 1])
+          else
+            next_level << level[i]
+          end
+          i += 2
+        end
+        level = next_level
+      end
+      level[0]
+    end
+
+    private def self.combine(a : Bytes, b : Bytes) : Bytes
+      d = OpenSSL::Digest.new("SHA256")
+      d.update(a)
+      d.update(b)
+      d.final
+    end
+  end
+end
diff --git a/PROJECTS/intermediate/credential-rotation-enforcer/src/cre/audit/signing.cr b/PROJECTS/intermediate/credential-rotation-enforcer/src/cre/audit/signing.cr
new file mode 100644
index 00000000..780111eb
--- /dev/null
+++ b/PROJECTS/intermediate/credential-rotation-enforcer/src/cre/audit/signing.cr
@@ -0,0 +1,116 @@
+# ===================
+# ©AngelaMos | 2026
+# signing.cr
+# ===================
+
+require "openssl"
+require "openssl/lib_crypto"
+
+lib LibCrypto
+  type EVP_PKEY = Void*
+
+  fun evp_pkey_new_raw_private_key_cre = EVP_PKEY_new_raw_private_key(type : LibC::Int, e : Void*, key : UInt8*, keylen : LibC::SizeT) : EVP_PKEY
+  fun evp_pkey_new_raw_public_key_cre = EVP_PKEY_new_raw_public_key(type : LibC::Int, e : Void*, key : UInt8*, keylen : LibC::SizeT) : EVP_PKEY
+  fun evp_pkey_get_raw_public_key_cre = EVP_PKEY_get_raw_public_key(pkey : EVP_PKEY, pub : UInt8*, len : LibC::SizeT*) : LibC::Int
+  fun evp_pkey_free_cre = EVP_PKEY_free(pkey : EVP_PKEY) : Void
+  fun evp_digestsigninit_cre = EVP_DigestSignInit(ctx : EVP_MD_CTX, pctx : Void*, type : EVP_MD, e : Void*, pkey : EVP_PKEY) : LibC::Int
+  fun evp_digestsign_cre = EVP_DigestSign(ctx : EVP_MD_CTX, sigret : UInt8*, siglen : LibC::SizeT*, tbs : UInt8*, tbslen : LibC::SizeT) : LibC::Int
+  fun evp_digestverifyinit_cre = EVP_DigestVerifyInit(ctx : EVP_MD_CTX, pctx : Void*, type : EVP_MD, e : Void*, pkey : EVP_PKEY) : LibC::Int
+  fun evp_digestverify_cre = EVP_DigestVerify(ctx : EVP_MD_CTX, sig : UInt8*, siglen : LibC::SizeT, tbs : UInt8*, tbslen : LibC::SizeT) : LibC::Int
+end
+
+module CRE::Audit::Signing
+  NID_ED25519         = 1087
+  ED25519_KEY_SIZE    =   32
+  ED25519_SIG_SIZE    =   64
+  ED25519_PUBKEY_SIZE =   32
+
+  class Error < OpenSSL::Error; end
+
+  class Ed25519Keypair
+    getter version : Int32
+    getter private_key : Bytes
+    getter public_key : Bytes
+
+    def initialize(@private_key : Bytes, @public_key : Bytes, @version : Int32)
+      raise ArgumentError.new("private key must be 32 bytes") unless @private_key.size == ED25519_KEY_SIZE
+      raise ArgumentError.new("public key must be 32 bytes") unless @public_key.size == ED25519_PUBKEY_SIZE
+    end
+
+    def self.generate(version : Int32 = 1) : Ed25519Keypair
+      private_key = ::Random::Secure.random_bytes(ED25519_KEY_SIZE)
+      pkey = LibCrypto.evp_pkey_new_raw_private_key_cre(NID_ED25519, Pointer(Void).null, private_key.to_unsafe, ED25519_KEY_SIZE.to_u64)
+      raise Error.new("EVP_PKEY_new_raw_private_key failed") if pkey.null?
+      begin
+        pubkey_buf = Bytes.new(ED25519_PUBKEY_SIZE)
+        len = ED25519_PUBKEY_SIZE.to_u64
+        rc = LibCrypto.evp_pkey_get_raw_public_key_cre(pkey, pubkey_buf.to_unsafe, pointerof(len))
+        raise Error.new("EVP_PKEY_get_raw_public_key failed (rc=#{rc})") unless rc == 1
+        Ed25519Keypair.new(private_key, pubkey_buf, version)
+      ensure
+        LibCrypto.evp_pkey_free_cre(pkey)
+      end
+    end
+  end
+
+  class Ed25519Signer
+    getter version : Int32
+
+    def initialize(@private_key : Bytes, @version : Int32)
+      raise ArgumentError.new("private key must be 32 bytes") unless @private_key.size == ED25519_KEY_SIZE
+    end
+
+    def self.from_keypair(kp : Ed25519Keypair) : Ed25519Signer
+      Ed25519Signer.new(kp.private_key, kp.version)
+    end
+
+    def sign(message : Bytes) : Bytes
+      pkey = LibCrypto.evp_pkey_new_raw_private_key_cre(NID_ED25519, Pointer(Void).null, @private_key.to_unsafe, ED25519_KEY_SIZE.to_u64)
+      raise Error.new("EVP_PKEY_new_raw_private_key failed") if pkey.null?
+      ctx = LibCrypto.evp_md_ctx_new
+      raise Error.new("EVP_MD_CTX_new failed") if ctx.null?
+      begin
+        rc = LibCrypto.evp_digestsigninit_cre(ctx, Pointer(Void).null, Pointer(Void).null.as(LibCrypto::EVP_MD), Pointer(Void).null, pkey)
+        raise Error.new("EVP_DigestSignInit failed (rc=#{rc})") unless rc == 1
+
+        siglen = ED25519_SIG_SIZE.to_u64
+        sig = Bytes.new(ED25519_SIG_SIZE)
+        rc = LibCrypto.evp_digestsign_cre(ctx, sig.to_unsafe, pointerof(siglen), message.to_unsafe, message.size.to_u64)
+        raise Error.new("EVP_DigestSign failed (rc=#{rc})") unless rc == 1
+
+        sig
+      ensure
+        LibCrypto.evp_md_ctx_free(ctx)
+        LibCrypto.evp_pkey_free_cre(pkey)
+      end
+    end
+  end
+
+  class Ed25519Verifier
+    def initialize(@public_key : Bytes)
+      raise ArgumentError.new("public key must be 32 bytes") unless @public_key.size == ED25519_PUBKEY_SIZE
+    end
+
+    def verify(message : Bytes, signature : Bytes) : Bool
+      return false unless signature.size == ED25519_SIG_SIZE
+
+      pkey = LibCrypto.evp_pkey_new_raw_public_key_cre(NID_ED25519, Pointer(Void).null, @public_key.to_unsafe, ED25519_PUBKEY_SIZE.to_u64)
+      return false if pkey.null?
+      ctx = LibCrypto.evp_md_ctx_new
+      if ctx.null?
+        LibCrypto.evp_pkey_free_cre(pkey)
+        return false
+      end
+      begin
+        rc = LibCrypto.evp_digestverifyinit_cre(ctx, Pointer(Void).null, Pointer(Void).null.as(LibCrypto::EVP_MD), Pointer(Void).null, pkey)
+        return false unless rc == 1
+
+        rc = LibCrypto.evp_digestverify_cre(ctx, signature.to_unsafe, signature.size.to_u64, message.to_unsafe, message.size.to_u64)
+        rc == 1
+      ensure
+        LibCrypto.evp_md_ctx_free(ctx)
+        LibCrypto.evp_pkey_free_cre(pkey)
+      end
+    end
+  end
+end