Recently, the vulnerability CVE-2025-22866 was identified in the assembly implementation of an internal cryptographic function on the ppc64le architecture (PowerPC 64-bit Little Endian). This vulnerability may cause a few bits of secret scalar values to leak due to timing differences in instruction execution. While the flaw does not appear able to expose private keys in standard cryptographic protocols (like ECDSA or TLS using the NIST P-256 curve), it’s a crucial reminder that even small timing differences can threaten cryptographic security.

Let's break down the issue in simple terms, walk through some code snippets that illustrate the root cause, and analyze the actual risk for users and systems.

What Happened: The Issue in Brief

In cryptography, timing side-channel attacks exploit differences in how long operations take to gain information about protected secrets. Ideally, code handling secrets runs in constant time — that is, every action takes the same amount of time, regardless of the secret data.

For CVE-2025-22866, a specific internal function in a cryptographic library — implemented in assembly for speed — uses a variable-time instruction on ppc64le CPUs. This allows an attacker who can measure how long the function takes to run to learn a few bits about secret values (called _scalars_), such as those in elliptic curve operations.

> While the function can leak a small number of bits, this is not enough (as used in common protocols and with P-256) to reconstruct the full private key.

Here's an illustrative (simplified) pseudo-assembly version of the problematic function

# Pseudo-assembly: Scalar multiplication inner loop
.loop:
    # Hypothetical variable-time instruction
    some_instruction r1, r2, r3   # Time depends on value of r3 (secret bits)
    # Other cryptographic logic

    cmp r3, 
    bne .loop

r3 holds bits of the secret scalar.

- An attacker measuring how long this function runs could infer (partly) the content of secret scalar bits.

Real code would look more complex, but this demonstrates the gist: _variable-time dependent on secret value = risky._

How Many Bits Leak?

This leak reveals a few bits of the secret scalar per execution (the exact number depends on implementation details of the instruction and the architecture). As a result, even after many observed signatures or exchanges, there's not enough information for an attacker to reconstruct the full private key.

Leak is tiny: Only a handful of bits leak per run.

- No single direct attack: You'd need to collect thousands to millions of operations to try and recover a key — and even then it’s unlikely.
- P-256 remains robust: With correct use of protocols, other countermeasures stop practical attacks.
- Not remotely triggerable: Attacker needs to precisely measure timings on the same hardware, or have a side-channel measurement (like on a shared VM host).

How Would An Attacker Try To Exploit This?

1. Gain precision timing: Either on the same machine (local attacker) or through shared hosting/cloud.
2. Trigger repeated cryptographic operations: E.g., make a server sign many things or run key exchanges.
3. Combine timing info: Get enough data to infer the leaked bits, and try to reconstruct the private scalar.

Example: Automated Timing Collection (Concept)

import time

def time_signature(private_key, message):
    start = time.perf_counter()
    signature = sign_message(private_key, message)
    end = time.perf_counter()
    return end - start

# Run thousands of times, process results

In the real world, real side-channel attacks get much more sophisticated — using cache or power measurements, and measuring sub-millisecond differences.

Official References

- CVE Page: NVD - CVE-2025-22866 *(link may be pending as CVE is recent)*
- OpenSSL Advisory: (If applicable) OpenSSL Security Advisories
- BoringSSL issue tracker: BoringSSL Issues
- Assembly constant-time best practices

Audit your platform: If you're not running on ppc64le, you’re not affected.

- Watch advisories: This class of timing side-channels pops up regularly — always update core cryptographic libraries promptly.
- Don’t panic: This leak is not exploitable in typical real-world use, but all cryptographic timing bugs should be fixed.

Conclusion

CVE-2025-22866 highlights the importance of constant-time coding, particularly in assembly where low-level instructions may act unpredictably on some architectures. While this issue is not a practical threat for P-256 users in day-to-day protocols, it shows why cryptography needs careful, architecture-specific code review and why keeping your libraries up to date is always a good idea.

*Stay safe — keep your crypto constant-time!*

*Want to learn more about side-channel attacks? Read Timing Attacks on Implementations of Diffie-Hellman, RSA, DSS, and Other Systems (1996) by Paul Kocher for the classic foundation.*

Timeline

Published on: 02/06/2025 17:15:21 UTC
Last modified on: 02/11/2025 15:15:20 UTC