diff --git a/README.md b/README.md
index 6d10058..1c5d735 100644
--- a/README.md
+++ b/README.md
@@ -138,8 +138,10 @@ random60:   0x03699c13c10128b5
 **Global lookup by public ID**
 GSI: `GSI1PK = PUBLICID#{orderlyid}`, `GSI1SK = CONST`
 
-**Postgres / MySQL**
-- Store both `id_text VARCHAR(64) UNIQUE` and `id_bin BINARY(20)`
+**SQL storage**
+- Store the text form as `id_text VARCHAR(69) UNIQUE`
+- Use `VARCHAR(64)` only if checksum suffixes are never stored
+- Store the 20-byte binary body as `id_bin BINARY(20)` on MySQL/Aurora or `id_bin BYTEA` on PostgreSQL
 - Index `id_bin` for range scans
 - Use `id_text` for APIs/logs
 
diff --git a/orderlyid_test.go b/orderlyid_test.go
index 4db6307..74e292a 100644
--- a/orderlyid_test.go
+++ b/orderlyid_test.go
@@ -54,7 +54,11 @@ func TestChecksumRoundTrip(t *testing.T) {
 		t.Fatalf("parse with checksum failed: %v", err)
 	}
 	// Tamper last char
-	bad := id[:len(id)-1] + "0"
+	repl := byte('0')
+	if id[len(id)-1] == repl {
+		repl = '1'
+	}
+	bad := id[:len(id)-1] + string(repl)
 	if _, err := Parse(bad); err == nil {
 		t.Fatalf("expected checksum mismatch")
 	} else if !errors.Is(err, ErrInvalidChecksum) {
diff --git a/spec/0001-spec.md b/spec/0001-spec.md
index 8c23a47..6bc21ea 100644
--- a/spec/0001-spec.md
+++ b/spec/0001-spec.md
@@ -54,23 +54,22 @@ OrderlyID provides typed, time-sortable, globally unique identifiers optimized f
 
 ## 6. Checksum
 
-1. `now_ms = system_clock_ms()`. If privacy enabled, quantize.  
-2. If another ID in the same ms: increment 12-bit seq; wrap allowed.  
-3. Draw 60 bits from CSPRNG.  
-4. Pack fields into 160-bit body, encode to 32 Base32 chars.  
-5. Emit `"<prefix>_<payload>"`; append checksum if configured.
-
-*Ordering semantics*: within same prefix, lexicographic sort ≈ `(time, flags, tenant, seq, shard, random)`.
+- Domain: HRP-expanded `"<prefix>_"` plus payload symbols.
+- Algorithm: Bech32 polymod, truncated to 20 bits and encoded as 4 Crockford Base32 characters.
+- Verification: if present, parsers MUST validate the checksum and reject mismatches.
+- False-accept probability: approximately 1 in 1,048,576.
+- The checksum is an integrity check for transcription errors, not cryptographic authentication.
 
 ## 7. Generation
 
-1. `now_ms = system_clock_ms()`. If privacy enabled, quantize.  
-2. If another ID in the same ms: increment 12-bit seq; wrap allowed.  
-3. Draw 60 bits from CSPRNG.  
-4. Pack fields into 160-bit body, encode to 32 Base32 chars.  
-5. Emit `"<prefix>_<payload>"`; append checksum if configured.
+1. `now_ms = system_clock_ms()`. If privacy bucketing is enabled, quantize to the configured bucket size.
+2. If another ID is generated in the same millisecond, increment the 12-bit `seq`; wrap is allowed.
+3. Draw 60 bits from a CSPRNG.
+4. Pack fields into the 160-bit body in big-endian order.
+5. Encode the body into exactly 32 Crockford Base32 characters.
+6. Emit `"<prefix>_<payload>"`; append `"-<checksum>"` if checksum is configured.
 
-*Ordering semantics*: within same prefix, lexicographic sort ≈ `(time, flags, tenant, seq, shard, random)`.
+*Ordering semantics*: within the same prefix, lexicographic sort ≈ `(time, flags, tenant, seq, shard, random)`.
 
 ---
 
@@ -96,7 +95,7 @@ OrderlyID provides typed, time-sortable, globally unique identifiers optimized f
 
 ## 10. Interop & Tests
 
-- **SQL**: store `id_text VARCHAR(64)` and `id_bin BINARY(20)`; index `id_bin` for range scans.  
+- **SQL**: store the text form as `id_text VARCHAR(69) UNIQUE`; use `VARCHAR(64)` only if checksum suffixes are never stored. Store the 20-byte binary body as `id_bin BINARY(20)` on MySQL/Aurora or `id_bin BYTEA` on PostgreSQL; index `id_bin` for range scans.
 - **DynamoDB**: use as sort key; shard global feeds via virtual partitions.  
 - **APIs/logs**: emit typed ID with checksum externally; redact tail if sensitive.  
 - **Conformance**: implementations claiming v1 MUST:
diff --git a/spec/README.md b/spec/README.md
index 69fb674..d645b09 100644
--- a/spec/README.md
+++ b/spec/README.md
@@ -94,7 +94,7 @@ checksum: 9xgg
 
 ## Interop & storage guidance
 
-- **SQL**: store both `id_text VARCHAR(64) UNIQUE` and `id_bin BINARY(20)`; index `id_bin` for range scans.
+- **SQL**: store the text form as `id_text VARCHAR(69) UNIQUE`; use `VARCHAR(64)` only if checksum suffixes are never stored. Store the 20-byte binary body as `id_bin BINARY(20)` on MySQL/Aurora or `id_bin BYTEA` on PostgreSQL; index `id_bin` for range scans.
 - **DynamoDB**: use ID as sort key; to avoid hot partitions, hash into virtual shards.
 - **APIs & logs**: prefer the typed public ID; redact tail chars if sensitive.