pkcs12: limit PBKDF iteration count to prevent CPU exhaustion

[mohammadmseet-hue]

The PKCS#12 PBKDF iteration count is read directly from the input file with no upper bound. A crafted .p12 file (83 bytes) with iterations set to MaxInt32 causes pkcs12.Decode() to block indefinitely.

Fix: Add a maximum iteration count of 1 million (well above typical values of 2048-10000) in both verifyMac and pbDecrypterFor, which are the two entry points that call pbkdf with the attacker-controlled iteration count.

Impact: An 83-byte crafted PKCS#12 file blocks pkcs12.Decode() permanently. Scaling: 1 iteration = 64µs, 10M iterations = 936ms, MaxInt32 = estimated 596,000+ hours.

Comparison: Other PBKDF implementations enforce similar limits:

• OpenSSL caps at PKCS5_MAX_PBKDF2_ITERATIONS (10M)
• Go's own x/crypto/scrypt validates its parameters
• Python's hashlib.pbkdf2_hmac has a built-in iteration limit

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Patch Set 1:

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