Manually Authenticated Diffie-Hellman

As explained in Real-World Cryptography

Manually Authenticated Diffie-Hellman is a key-exchange technique that adds a commitment step and human comparison to hinder active man-in-the-middle attacks.

Manually Authenticated Diffie-Hellman (MA-DH) strengthens a Diffie-Hellman key exchange against an active man-in-the-middle attacker. Devices compare a short authenticated string (SAS), but first commit to public keys so neither side can choose a key after seeing the other’s. section 11.3.3

A secure SAS-based exchange works in three steps:

• Alice sends a commitment to her public key. section 11.3.3
• Alice receives Bob’s public key, then reveals her committed public key. section 11.3.3
• Both devices derive an SAS and the user compares the displayed values; matching values indicate that the exchange was not MITM’d. section 11.3.3

Pairing a phone with a car

Bluetooth Numeric Comparison can display an SAS during pairing. The short value is convenient for real-time comparison, unlike a long fingerprint, though Bluetooth security was noted as broken by the KNOB attack as of 2021. section 11.3.3 section 11.3.3

Figure 11.17 The diagram on the left pictures a secure SAS-based protocol in which Alice first sends a commitment of her public key. She then only reveals her public key after receiving Bob’s public key. As she committed to it, she cannot freely choose her key pair based on Bob’s key. If the exchange is actively MITM’d (diagram on the right), the attacker cannot choose either key pairs to force Alice’s and Bob’s SAS to match.

Shows how committing before revelation prevents an attacker from selecting public keys that force both devices to display the same SAS. section 11.3.3

How Real-World Cryptography covers it

How deep it goes

Explains SAS weakness and the commitment defense, but does not provide implementation details or a formal security proof.

Examples

Uses Bluetooth Numeric Comparison, phone-to-car pairing, public-key substitution, and SAS matching as examples.

What it assumes

Assumes familiarity with public keys, hashing, and unauthenticated key exchange.

What you can do after

Evaluate whether a SAS-based pairing protocol limits active man-in-the-middle key selection.

How it connects

Connects Diffie-Hellman, short authenticated strings, public-key commitments, and man-in-the-middle attacks.

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Related concepts

• key exchangekind of · Key exchange is a cryptographic protocol by which communicating parties establish a shared secret over a communication channel without directly transmitting that secret.

This explainer was generated by Manning's knowledge graph from the text of Real-World Cryptography. The section links point to the parts of the book it draws on.