Disallow writing smaller manifest into asset than allocated for

For compliant validators, this would cause a hash mismatch (#68)

src/asset/BMFF.ts

@@ -106,7 +106,7 @@ export class BMFF extends BaseAsset implements Asset {
 
     public async ensureManifestSpace(length: number): Promise<void> {
         // Nothing to do?
-        if (((this.getManifestStoreBox()?.payload as C2PAManifestBoxPayload)?.manifestContent.length ?? 0) >= length)
+        if (((this.getManifestStoreBox()?.payload as C2PAManifestBoxPayload)?.manifestContent.length ?? 0) === length)
             return;
 
         const parts: {
@@ -160,8 +160,8 @@ export class BMFF extends BaseAsset implements Asset {
 
     public async writeManifestJUMBF(jumbf: Uint8Array): Promise<void> {
         const box = this.getManifestStoreBox();
-        if (!box || (box.payload as C2PAManifestBoxPayload).manifestContent.length < jumbf.length)
-            throw new Error('Not enough space in asset file');
+        if (!box || (box.payload as C2PAManifestBoxPayload).manifestContent.length !== jumbf.length)
+            throw new Error('Wrong amount of space in asset');
 
         box.fillManifestContent(this.data, jumbf);
     }
@@ -656,13 +656,16 @@ class C2PABox extends FullBox<C2PABoxPayload> {
      */
     public fillManifestContent(buf: Uint8Array, manifest: Uint8Array): void {
         const payload = this.payload as C2PAManifestBoxPayload;
-        payload.manifestContent.fill(0);
         payload.manifestContent.set(manifest);
 
         const dataView = new DataView(buf.buffer, this.payloadOffset, this.payloadSize);
+        // Write purpose string
         payload.purpose.split('').forEach((c, i) => dataView.setUint8(i, c.charCodeAt(0)));
+        // Write null terminator
         dataView.setUint8(payload.purpose.length, 0);
+        // Write Merkle offset
         dataView.setBigUint64(payload.purpose.length + 1, payload.merkleOffset);
-        buf.set(payload.manifestContent, this.payloadOffset + payload.purpose.length + 9);
+        // Write content
+        buf.set(manifest, this.payloadOffset + payload.purpose.length + 9);
     }
 }

src/asset/JPEG.ts

@@ -283,7 +283,6 @@ export class JPEG extends BaseAsset implements Asset {
     }
 
     public async writeManifestJUMBF(jumbf: Uint8Array): Promise<void> {
-        // For JPEG, the segments' payload length needs to match the JUMBF length exactly
         if (this.getJUMBFLength(this.manifestSegments) !== jumbf.length)
             throw new Error('Wrong amount of space in asset');
 

src/asset/PNG.ts

@@ -100,8 +100,8 @@ export class PNG extends BaseAsset implements Asset {
     }
 
     public async ensureManifestSpace(length: number): Promise<void> {
-        // Existing manifest chunk is already large enough?
-        if (this.manifestChunkIndex !== undefined && this.chunks[this.manifestChunkIndex].payloadLength >= length)
+        // Nothing to do?
+        if (this.manifestChunkIndex !== undefined && this.chunks[this.manifestChunkIndex].payloadLength === length)
             return;
 
         // Ensure there is a manifest chunk in the list of chunks
@@ -156,20 +156,15 @@ export class PNG extends BaseAsset implements Asset {
     public async writeManifestJUMBF(jumbf: Uint8Array): Promise<void> {
         if (
             this.manifestChunkIndex === undefined ||
-            this.chunks[this.manifestChunkIndex].payloadLength < jumbf.length
+            this.chunks[this.manifestChunkIndex].payloadLength !== jumbf.length
         ) {
-            throw new Error('Not enough space in asset file');
+            throw new Error('Wrong amount of space in asset');
         }
 
         const manifestChunk = this.chunks[this.manifestChunkIndex];
         const dataBuffer = manifestChunk.getSubBuffer(this.data);
         dataBuffer.set(jumbf);
 
-        // If the chunk is larger than the manifest, zero out the remainder
-        if (manifestChunk.payloadLength > jumbf.length) {
-            dataBuffer.fill(0, jumbf.length);
-        }
-
         // Update CRC
         manifestChunk.updateCRC(this.data);
         new DataView(this.data.buffer, manifestChunk.offset).setUint32(manifestChunk.length - 4, manifestChunk.crc);