fix(kerberos): recover from KDC clock skew during AS exchange

[Marc-André Moreau (mamoreau-devolutions)]

Summary

• When a KDC rejects encrypted AS pre-authentication with KRB_AP_ERR_SKEW, derive a per-context time offset from the error's stime/susec and retry once. Propagate a second skew error or any other error without additional retries.
• Apply the offset to password, keytab, and smart-card pre-authentication timestamps and to subsequent TGS, AP, and password-change authenticators. Existing public generator calls still use local time unless invoked through the corrected client context.
• Fix the built-in KDC's skew check so timestamps slightly ahead of or behind its clock are accepted within max_time_skew.

Review and validation

• End-to-end tests cover KDC clocks 15 seconds ahead and behind, a bounded second skew error, and an unrelated pre-authentication failure with no retry. A KDC test verifies both directions inside/outside the configured skew window; a keytab test decrypts and checks the encoded timestamp.
• cargo test -q -p sspi -p kdc --features network_client,__test-data
• cargo check -q -p sspi --all-features
• cargo clippy -q -p sspi -p kdc --features network_client,__test-data --lib --tests -- -D warnings -A unreachable-pub -A clippy::non-ascii-literal (the two allowances are for pre-existing warnings outside this change)
• cargo fmt --all

Scope: This addresses KDC AS pre-authentication skew. There is no trace from the original PSWSMan report to prove it is the exact failure; a separately skewed application server rejecting the AP exchange is not handled here (related: #83).

[Copilot]

Copilot review overview

🟢 Approval recommended

The retry is bounded, error propagation is preserved, and the affected authentication paths have focused validation.

Review effort: Balanced
Findings: None

What changed in this PR

Adds per-context KDC clock-skew recovery during Kerberos AS pre-authentication and propagates corrected time to subsequent authenticators.

Changes:

• Retries AS pre-authentication once after KRB_AP_ERR_SKEW.
• Applies learned offsets across password, keytab, smart-card, TGS, AP, and password-change flows.
• Corrects bidirectional KDC skew validation and adds tests.

File	Description
tests/​sspi/​client_server/​kerberos/​mod.rs	Tests recovery, retry limits, and unrelated errors.
tests/​sspi/​client_server/​kerberos/​kdc.rs	Adds configurable mock clock skew.
src/​pk_init.rs	Supports supplied smart-card timestamps.
src/​kerberos/​tests.rs	Initializes the new context field.
src/​kerberos/​pa_datas.rs	Routes corrected time through pre-auth generation.
src/​kerberos/​mod.rs	Stores and applies the per-context offset.
src/​kerberos/​client/​mod.rs	Uses corrected time for TGS and AP authenticators.
src/​kerberos/​client/​generators.rs	Adds timestamp-aware generators and keytab coverage.
src/​kerberos/​client/​change_password.rs	Corrects password-change authenticator time.
src/​kerberos/​client/​as_exchange.rs	Parses skew errors and performs one retry.
crates/​kdc/​src/​as_exchange.rs	Accepts skew within the configured window in either direction.

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[Pavlo Myroniuk (TheBestTvarynka)]

LGTM