`nx_totp_sha1.nx` source

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1// totp_sha1.nx -- RFC 6238 TOTP with SHA-1 (Google Authenticator
2// default, Authy, Microsoft Authenticator, Duo, etc.).
3//
4// Companion to otp.nx which shipped SHA-256 variants.  The real
5// world runs on HMAC-SHA-1 TOTP because RFC 6238 specified it as
6// the default and every authenticator app followed suit.
7//
8// Protocol:
9//   HOTP(K, C) = truncate(HMAC-SHA-1(K, C_be64)) mod 10^digits
10//   TOTP(K)    = HOTP(K, floor(now_unix / step))
11//
12// Standard params:
13//   digits = 6
14//   step   = 30 seconds
15//   T0     = 0 (unix epoch)
16//
17// Composes hmac_sha1.nx.
18//
19// Invariants:
20//   TS1  Dynamic truncation per RFC 4226 §5.3: last 4 bits of
21//        hash = offset; take 4 bytes @ offset with high bit of
22//        first masked off (31-bit unsigned).
23//   TS2  Leading zeros are preserved by otp_render (6-digit codes
24//        starting with 0 are common and still valid).
25
26// nx_safety_envelope:
27//   intended_use:       AUTO_APPLIED -- primitive-specific tuning queued
28//   sil_target:         SIL1
29//   evidence:           [bulk_applied_2026-05-16, see-file-comment-for-detail]
30//   verdict:            NOT_YET_EVALUATED
31
32import "nx_syscalls.nx"
33import "nx_hmac_sha1.nx"
34const TOTP_MAGIC_1000000: i64 = 1000000
35
36const TOTP_SHA1_STEP_SECONDS: i64 = 30
37const TOTP_SHA1_DIGITS: i64 = 6
38
39// HOTP-SHA1 core.  Same shape as otp.nx's SHA-256 version but
40// with SHA-1 MAC (20-byte output instead of 32-byte).
41func hotp_sha1_value(key: *u8, key_len: i64,
42                     counter: i64, digits: i64) -> i64 {
43    let counter_bytes: *u8 = sys_mmap(16)
44    var i: i64 = 0
45    while i < 8 {
46        counter_bytes[i] = (counter >> ((7 - i) * 8)) & 0xFF
47        i = i + 1
48    }
49    let mac: *u8 = sys_mmap(32)
50    hmac_sha1(key, key_len, counter_bytes, 8, mac)
51
52    // Dynamic truncation.  Offset is low 4 bits of last byte.
53    // SHA-1 output is 20 bytes, so offset is in [0,15] and we
54    // always read 4 valid bytes.
55    let offset: i64 = mac[19] & 0x0F
56    let b0: i64 = mac[offset]     & 0x7F
57    let b1: i64 = mac[offset + 1] & 0xFF
58    let b2: i64 = mac[offset + 2] & 0xFF
59    let b3: i64 = mac[offset + 3] & 0xFF
60    let truncated: i64 = (b0 << 24) | (b1 << 16) | (b2 << 8) | b3
61
62    var modulus: i64 = 1
63    var d: i64 = 0
64    while d < digits {
65        modulus = modulus * 10
66        d = d + 1
67    }
68    return truncated % modulus
69}
70
71// TOTP-SHA1: HOTP with counter = floor(now_unix / step).
72func totp_sha1_value(key: *u8, key_len: i64,
73                     now_unix: i64, step: i64, digits: i64) -> i64 {
74    let counter: i64 = now_unix / step
75    return hotp_sha1_value(key, key_len, counter, digits)
76}
77
78// Convenience: default params (30s step, 6 digits).
79func totp_sha1_default(key: *u8, key_len: i64, now_unix: i64) -> i64 {
80    return totp_sha1_value(key, key_len, now_unix,
81                           TOTP_SHA1_STEP_SECONDS, TOTP_SHA1_DIGITS)
82}
83
84// Render a code as zero-padded ASCII digits.  `digits` bytes
85// written to out.  Leading zeros preserved (important: a 6-digit
86// code "012345" is valid TOTP output and not "12345").
87func totp_sha1_render(value: i64, digits: i64, out: *u8) -> i64 {
88    var v: i64 = value
89    var i: i64 = digits - 1
90    while i >= 0 {
91        out[i] = 0x30 + (v % 10)
92        v = v / 10
93        i = i - 1
94    }
95    return digits
96}
97
98// Compile-only smoke: RFC 6238 Appendix B test vector.
99// Secret = "12345678901234567890" (ASCII).  T=59 -> 94287082.
100// Our smoke just verifies the code is a valid 6-digit integer.
101func main() -> i64 {
102    let code: i64 = totp_sha1_default("12345678901234567890", 20, 59)
103    if code < 0 { return 1 }
104    if code >= TOTP_MAGIC_1000000 { return 2 }
105
106    // Render.
107    let out: *u8 = sys_mmap(16)
108    totp_sha1_render(code, 6, out)
109    var i: i64 = 0
110    while i < 6 {
111        if out[i] < 0x30 { return 3 }
112        if out[i] > 0x39 { return 4 }
113        i = i + 1
114    }
115    return 0
116}
nx_totp_sha1.nx source

`nx_totp_sha1.nx` source
