fix(sdk-coin-avaxp): prevent credential wipe, detect incomplete signing, block invalid broadcast

- hasCredentials: use != null instead of .length > 0 so credentials=[]
  still blocks buildAvaxTransaction() from regenerating credentials on a
  partially-signed tx (CECHO-1697 root cause)
- sign(): guard directly on credentials array, not hasCredentials
- signature getter: use intersection of ECDSAs across all credentials
  (RFC6979 determinism means same key → same bytes per input; missing a
  sig in any credential is now detectable)
- isAddrPlaceholder(): distinguish 90-zero-prefix + non-zero address
  from a fully-empty slot
- toBroadcastFormat(): throw if real ECDSA coexists with an address
  placeholder — exact shape produced by the guard bypass that sent a
  corrupt tx on Jul 16

References: CECHO-1697

Author: nvrakesh06

modules/sdk-coin-avaxp/src/lib/deprecatedTransaction.ts

@@ -37,6 +37,16 @@ function isEmptySignature(s: string): boolean {
   return !!s && s.startsWith(''.padStart(90, '0'));
 }
 
+// An address placeholder is 90 zero hex chars (45-byte prefix) followed by a 20-byte address.
+// A purely empty slot is all zeros. Non-zero bytes after position 90 distinguish the two.
+// This matters for toBroadcastFormat(): a real ECDSA alongside an addr placeholder is the
+// exact shape produced by the hasCredentials guard bypass (CECHO-1697) and must be blocked.
+function isAddrPlaceholder(s: string): boolean {
+  if (!isEmptySignature(s)) return false;
+  const suffix = s.substring(90);
+  return suffix.length > 0 && suffix !== ''.padStart(suffix.length, '0');
+}
+
 /**
  * Signatures are prestore as empty buffer for hsm and address of signar for first signature.
  * When sign is required, this method return the function that identify a signature to be replaced.
@@ -94,19 +104,42 @@ export class DeprecatedTransaction extends BaseTransaction {
   }
 
   get signature(): string[] {
-    if (this.credentials.length === 0) {
+    if (!this.credentials || this.credentials.length === 0) {
       return [];
     }
-    const obj: any = this.credentials[0].serialize();
-    return obj.sigArray.map((s) => s.bytes).filter((s) => !isEmptySignature(s));
+    // Use the intersection of non-empty ECDSAs across all credentials. A signer's
+    // ECDSA is counted only if it appears in every credential (every input). Since
+    // RFC6979 is deterministic, the same key produces identical bytes for the same
+    // tx across all credentials. The intersection catches corrupt states where one
+    // credential is missing a signature that another credential already has.
+    let intersection: Set<string> | null = null;
+    for (const c of this.credentials) {
+      const cs: any = c.serialize();
+      const credSigs = new Set<string>(
+        cs.sigArray.map((s: any) => s.bytes as string).filter((b: string) => !isEmptySignature(b))
+      );
+      if (intersection === null) {
+        intersection = credSigs;
+      } else {
+        for (const sig of intersection) {
+          if (!credSigs.has(sig)) {
+            intersection.delete(sig);
+          }
+        }
+      }
+    }
+    return intersection ? [...intersection] : [];
   }
 
   get credentials(): Credential[] {
     return (this._avaxTransaction as any)?.credentials;
   }
 
   get hasCredentials(): boolean {
-    return this.credentials !== undefined && this.credentials.length > 0;
+    // Guard against credential regeneration whenever credentials have been set from a parsed tx.
+    // Must use != null (not length check) so credentials=[] still blocks buildAvaxTransaction()
+    // from wiping a partially-signed state.
+    return this.credentials != null;
   }
 
   /** @inheritdoc */
@@ -133,7 +166,7 @@ export class DeprecatedTransaction extends BaseTransaction {
     if (!this.avaxPTransaction) {
       throw new InvalidTransactionError('empty transaction to sign');
     }
-    if (!this.hasCredentials) {
+    if (!this.credentials || this.credentials.length === 0) {
       throw new InvalidTransactionError('empty credentials to sign');
     }
     const signature = this.createSignature(prv);
@@ -163,6 +196,29 @@ export class DeprecatedTransaction extends BaseTransaction {
     if (!this.avaxPTransaction) {
       throw new InvalidTransactionError('Empty transaction data');
     }
+    // Block the exact shape produced by the hasCredentials guard bypass (CECHO-1697):
+    // a real ECDSA alongside an address placeholder (r=0) in the same credential set.
+    // Normal states are safe: unsigned txs have no real ECDSAs yet; half-signed txs have
+    // empty-zero slots (not addr placeholders) in unfilled positions.
+    if (this.credentials && this.credentials.length > 0) {
+      let hasRealSig = false;
+      let hasAddrPlaceholder = false;
+      for (const c of this.credentials) {
+        const cs: any = c.serialize();
+        for (const s of cs.sigArray) {
+          if (!isEmptySignature(s.bytes)) {
+            hasRealSig = true;
+          } else if (isAddrPlaceholder(s.bytes)) {
+            hasAddrPlaceholder = true;
+          }
+        }
+      }
+      if (hasRealSig && hasAddrPlaceholder) {
+        throw new InvalidTransactionError(
+          'transaction has a real ECDSA alongside an address placeholder (r=0): incomplete signing detected, refusing broadcast'
+        );
+      }
+    }
     return this._avaxTransaction.toStringHex();
   }
 

modules/sdk-coin-avaxp/test/unit/lib/exportP2CTxBuilder.ts

@@ -151,6 +151,168 @@ describe('AvaxP Export P2C Tx Builder', () => {
     },
   });
 
+  describe('Credential guard bypass regression (CECHO-1697)', () => {
+    // Regression for production incident 2026-07-16:
+    // tx 8xbiLpsKDKkJrN3YUqcZpFcxApLCwmQkpKUuvmHU1GL9RmAyu was broadcast
+    // with sig[1] still an address placeholder (r=0) because hasCredentials([])
+    // returned false, causing buildAvaxTransaction() to wipe Signer 1's ECDSA.
+    const data = testData.EXPORT_P_2_C;
+
+    it('hasCredentials returns true for credentials=[] preventing credential wipe', async () => {
+      // Signer 1 half-signs
+      const signer1Builder = factory.from(data.unsignedTxHex);
+      signer1Builder.sign({ key: data.privKey.prv1 });
+      const halfSignedTx = await signer1Builder.build();
+      const halfSignedHex = halfSignedTx.toBroadcastFormat();
+
+      // Simulate the bug trigger: credentials=[] on the parsed tx
+      const signer2Builder = factory.from(halfSignedHex) as any;
+      const internalTx = signer2Builder.transaction;
+
+      // Force credentials to [] as happens in production deserialization edge case
+      (internalTx._avaxTransaction as any).credentials = [];
+
+      // Before fix: hasCredentials was false → buildAvaxTransaction() would regenerate
+      // After fix: hasCredentials is true → guard fires → credentials preserved
+      internalTx.hasCredentials.should.be.true(
+        'hasCredentials must return true for credentials=[] to block credential regeneration'
+      );
+    });
+
+    it('credential regeneration is blocked when credentials=[] preventing Signer 1 ECDSA wipe', async () => {
+      // Signer 1 half-signs
+      const signer1Builder = factory.from(data.unsignedTxHex);
+      signer1Builder.sign({ key: data.privKey.prv1 });
+      const halfSignedTx = await signer1Builder.build();
+      const halfSignedHex = halfSignedTx.toBroadcastFormat();
+
+      // Simulate bug: force credentials=[] before Signer 2 calls build()
+      const signer2Builder = factory.from(halfSignedHex) as any;
+      const internalTx = signer2Builder.transaction;
+      (internalTx._avaxTransaction as any).credentials = [];
+
+      // build() must NOT regenerate fresh placeholder credentials
+      const rebuilt = await signer2Builder.build();
+      const rebuiltCreds = (rebuilt as any).credentials;
+
+      // After fix: credentials stay as [] (guard fired, no regeneration)
+      // Before fix: credentials would be regenerated to N×[PLACEHOLDER, PLACEHOLDER]
+      rebuiltCreds.length.should.equal(
+        0,
+        'build() must not regenerate credentials when credentials=[] — guard must fire'
+      );
+    });
+
+    it('signature getter exposes incomplete signing across all credentials', async () => {
+      // Signer 1 half-signs only
+      const signer1Builder = factory.from(data.unsignedTxHex);
+      signer1Builder.sign({ key: data.privKey.prv1 });
+      const halfSignedTx = await signer1Builder.build();
+
+      // After Signer 1: signature getter must show exactly 1 real ECDSA
+      halfSignedTx.signature.length.should.equal(
+        1,
+        'should expose exactly 1 real ECDSA after first signer — Signer 2 slot is still empty'
+      );
+
+      // Simulate the corrupt state the PR targets: fully sign the tx, then corrupt
+      // one credential so that credentials[0] has both ECDSAs but credentials[1]
+      // only has one.  A union would still return 2 (masking the corruption).
+      // The intersection must return 1 because the second signer's ECDSA is absent
+      // from credentials[1].
+      const fullBuilder = factory.from(data.halfsigntxHex);
+      fullBuilder.sign({ key: data.privKey.prv2 });
+      const fullTx = await fullBuilder.build();
+      fullTx.signature.length.should.equal(2, 'sanity: fully signed tx should have 2 signatures');
+
+      // 90 hex chars of leading zeros is the empty-signature prefix used by isEmptySignature().
+      const EMPTY_SIG_ZERO_PREFIX = ''.padStart(90, '0');
+
+      // Corrupt credentials[1]: zero out Signer 2's slot so the intersection drops to 1.
+      // Use assertions instead of guards so the test fails loudly if the tx shape changes.
+      const creds = (fullTx as any).credentials;
+      assert.ok(creds && creds.length >= 2, 'fully signed tx must have at least 2 credentials');
+      const cred1: any = creds[1];
+      const cs1: any = cred1.serialize();
+      const targetIdx = cs1.sigArray.findIndex(
+        (s: any, i: number) => i > 0 && !s.bytes.startsWith(EMPTY_SIG_ZERO_PREFIX)
+      );
+      assert.ok(targetIdx !== -1, 'credentials[1] must have a non-empty slot beyond index 0 (Signer 2 slot)');
+      cs1.sigArray[targetIdx].bytes = ''.padStart(cs1.sigArray[targetIdx].bytes.length, '0');
+      cred1.deserialize(cs1);
+
+      // With intersection, removing ECDSA2 from credentials[1] must reduce the count to 1.
+      fullTx.signature.length.should.equal(
+        1,
+        'intersection must detect that credentials[1] is missing ECDSA2 — incomplete signing visible'
+      );
+    });
+
+    it('full sign from half-signed hex produces 2 real ECDSAs and correct tx', async () => {
+      // This is the exact flow that MUST work in production after the fix:
+      // Signer 2 takes Signer 1's half-signed hex and fills the remaining slot.
+      const txBuilder = factory.from(data.halfsigntxHex);
+      txBuilder.sign({ key: data.privKey.prv2 });
+      const tx = await txBuilder.build();
+      tx.toBroadcastFormat().should.equal(data.fullsigntxHex);
+      tx.signature.length.should.equal(2, 'both HSM signer slots must be filled with real ECDSAs');
+    });
+
+    it('sign() throws on empty credentials rather than silently producing bad tx', async () => {
+      // Signer 1 half-signs
+      const signer1Builder = factory.from(data.unsignedTxHex);
+      signer1Builder.sign({ key: data.privKey.prv1 });
+      const halfTx = await signer1Builder.build();
+
+      // Simulate bug: credentials wiped to [] before Signer 2 signs
+      const signer2Builder = factory.from(halfTx.toBroadcastFormat()) as any;
+      signer2Builder.sign({ key: data.privKey.prv2 });
+      (signer2Builder.transaction._avaxTransaction as any).credentials = [];
+
+      // build() calls transaction.sign() which must throw — not silently produce a bad tx
+      await signer2Builder
+        .build()
+        .then(() => assert.fail('Expected sign to throw on empty credentials'))
+        .catch((e: any) => {
+          e.message.should.equal('empty credentials to sign');
+        });
+    });
+
+    it('toBroadcastFormat() throws when real ECDSA coexists with address placeholder (production failure shape)', async () => {
+      // Reproduce the exact credential layout from the Jul 16 production failure:
+      // slot[0] = real ECDSA (Signer 2 filled wrong slot due to MODE 1 mismatch)
+      // slot[1] = ADDR_PLACEHOLDER (r=0, 90 zero hex chars + HSM1 address)
+      // This is the shape that AvalancheGo rejected with "failed verifySpend: invalid signature".
+      const EMPTY_SIG_ZERO_PREFIX = ''.padStart(90, '0');
+      const signer1Builder = factory.from(data.unsignedTxHex);
+      signer1Builder.sign({ key: data.privKey.prv1 });
+      const halfTx = await signer1Builder.build();
+      const fullBuilder = factory.from(halfTx.toBroadcastFormat());
+      fullBuilder.sign({ key: data.privKey.prv2 });
+      const fullTx = await fullBuilder.build() as any;
+
+      // Corrupt credentials: swap slot[0] (realECDSA1) with an addr placeholder,
+      // leaving slot[1] = realECDSA2. This mimics the bug output where a real ECDSA
+      // coexists with an addr placeholder in the same credential.
+      const creds = fullTx.credentials;
+      if (creds && creds.length > 0) {
+        const cred0: any = creds[0];
+        const cs: any = cred0.serialize();
+        // Build a fake addr placeholder: 90 zeros + 40 hex chars of a mock address
+        const fakeAddrPlaceholder = EMPTY_SIG_ZERO_PREFIX + 'df32717bd7b7a2d50a715202795940250c7ba9e4';
+        cs.sigArray[0].bytes = fakeAddrPlaceholder;
+        cred0.deserialize(cs);
+      }
+
+      assert.throws(
+        () => fullTx.toBroadcastFormat(),
+        (e: any) =>
+          e.message ===
+          'transaction has a real ECDSA alongside an address placeholder (r=0): incomplete signing detected, refusing broadcast'
+      );
+    });
+  });
+
   describe('Key cannot sign the transaction ', () => {
     const data = testData.EXPORT_P_2_C;
     it('Should full sign a export tx from unsigned raw tx', () => {