Haiku

Claude Encrypted Thinking Blocks Use Protobuf with Exposed Metadata and AES-GCM Ciphertext

A detailed reverse-engineering of Claude signatures shows that the encrypted reasoning blocks are not opaque containers but structured protobuf messages. The outer envelope contains a 312-byte inner message that holds a 135-byte header, fixed-length nonce and MAC fields, and the actual ciphertext. The header itself reveals the model name such as claude-opus-5, the block type as thinking, and the organizationUuid from the user's Anthropic account. Only the reasoning text is encrypted with AES-GCM, adding exactly 16 bytes for the authentication tag. The analysis covers four protocol versions and notes that organization binding was added in version 15, potentially allowing servers to reject cross-model or cross-organization reuse. The findings provide concrete implications for both the Opus-to-Haiku extraction attack and the leakage of account identifiers in public logs.