bt_rfcomm/frame/
fcs.rs

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1// Copyright 2020 The Fuchsia Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5// CRC table for calculating FCS. GSM 7.10 Annex B, Section B.3.5.
6const CRC_TABLE: [u8; 256] = [
7    0x00, 0x91, 0xE3, 0x72, 0x07, 0x96, 0xE4, 0x75, 0x0E, 0x9F, 0xED, 0x7C, 0x09, 0x98, 0xEA, 0x7B,
8    0x1C, 0x8D, 0xFF, 0x6E, 0x1B, 0x8A, 0xF8, 0x69, 0x12, 0x83, 0xF1, 0x60, 0x15, 0x84, 0xF6, 0x67,
9    0x38, 0xA9, 0xDB, 0x4A, 0x3F, 0xAE, 0xDC, 0x4D, 0x36, 0xA7, 0xD5, 0x44, 0x31, 0xA0, 0xD2, 0x43,
10    0x24, 0xB5, 0xC7, 0x56, 0x23, 0xB2, 0xC0, 0x51, 0x2A, 0xBB, 0xC9, 0x58, 0x2D, 0xBC, 0xCE, 0x5F,
11    0x70, 0xE1, 0x93, 0x02, 0x77, 0xE6, 0x94, 0x05, 0x7E, 0xEF, 0x9D, 0x0C, 0x79, 0xE8, 0x9A, 0x0B,
12    0x6C, 0xFD, 0x8F, 0x1E, 0x6B, 0xFA, 0x88, 0x19, 0x62, 0xF3, 0x81, 0x10, 0x65, 0xF4, 0x86, 0x17,
13    0x48, 0xD9, 0xAB, 0x3A, 0x4F, 0xDE, 0xAC, 0x3D, 0x46, 0xD7, 0xA5, 0x34, 0x41, 0xD0, 0xA2, 0x33,
14    0x54, 0xC5, 0xB7, 0x26, 0x53, 0xC2, 0xB0, 0x21, 0x5A, 0xCB, 0xB9, 0x28, 0x5D, 0xCC, 0xBE, 0x2F,
15    0xE0, 0x71, 0x03, 0x92, 0xE7, 0x76, 0x04, 0x95, 0xEE, 0x7F, 0x0D, 0x9C, 0xE9, 0x78, 0x0A, 0x9B,
16    0xFC, 0x6D, 0x1F, 0x8E, 0xFB, 0x6A, 0x18, 0x89, 0xF2, 0x63, 0x11, 0x80, 0xF5, 0x64, 0x16, 0x87,
17    0xD8, 0x49, 0x3B, 0xAA, 0xDF, 0x4E, 0x3C, 0xAD, 0xD6, 0x47, 0x35, 0xA4, 0xD1, 0x40, 0x32, 0xA3,
18    0xC4, 0x55, 0x27, 0xB6, 0xC3, 0x52, 0x20, 0xB1, 0xCA, 0x5B, 0x29, 0xB8, 0xCD, 0x5C, 0x2E, 0xBF,
19    0x90, 0x01, 0x73, 0xE2, 0x97, 0x06, 0x74, 0xE5, 0x9E, 0x0F, 0x7D, 0xEC, 0x99, 0x08, 0x7A, 0xEB,
20    0x8C, 0x1D, 0x6F, 0xFE, 0x8B, 0x1A, 0x68, 0xF9, 0x82, 0x13, 0x61, 0xF0, 0x85, 0x14, 0x66, 0xF7,
21    0xA8, 0x39, 0x4B, 0xDA, 0xAF, 0x3E, 0x4C, 0xDD, 0xA6, 0x37, 0x45, 0xD4, 0xA1, 0x30, 0x42, 0xD3,
22    0xB4, 0x25, 0x57, 0xC6, 0xB3, 0x22, 0x50, 0xC1, 0xBA, 0x2B, 0x59, 0xC8, 0xBD, 0x2C, 0x5E, 0xCF,
23];
24
25/// Returns the FCS calculation for the provided byte `buf`.
26/// This calculation is derived exactly as specified in GSM 7.10 Section 5.2.1.6.
27/// See GSM 7.10 Section B.3.3 for the algorithm.
28pub fn calculate_fcs(buf: &[u8]) -> u8 {
29    let mut fcs: u8 = 0xFF;
30    let mut offset: usize = 0;
31    while offset != buf.len() {
32        let idx: usize = (fcs ^ buf[offset]).into();
33        fcs = CRC_TABLE[idx];
34        offset += 1;
35    }
36    // Ones complement.
37    0xFF - fcs
38}
39
40/// Given the `fcs` value and the associated `buf`, returns true if the Frame Check Sequence
41/// calculation is valid for the buffer.
42/// This calculation is derived exactly as specified in GSM 7.10 Section 5.2.1.6.
43/// See GSM 7.10 Section B.3.4 for the algorithm.
44pub fn verify_fcs(fcs: u8, buf: &[u8]) -> bool {
45    let mut calculated_fcs: u8 = 0xFF;
46    let mut offset: usize = 0;
47    while offset != buf.len() {
48        let idx: usize = (calculated_fcs ^ buf[offset]).into();
49        calculated_fcs = CRC_TABLE[idx];
50        offset += 1;
51    }
52    let idx: usize = (calculated_fcs ^ fcs).into();
53    calculated_fcs = CRC_TABLE[idx];
54    return calculated_fcs == 0xCF;
55}
56
57#[cfg(test)]
58mod tests {
59    use super::*;
60
61    #[test]
62    fn test_calculate_and_verify_fcs_sabm() {
63        let buf = vec![
64            0b00010111, // Address Field - EA = 1, C/R = 1, DLCI = 5.
65            0b00101111, // Control Field - SABM command with P/F = 0.
66            0b00000001, // Length Field - Bit1 = 1 Indicates one octet length - no info.
67        ];
68
69        let fcs = calculate_fcs(&buf[..]);
70        assert!(verify_fcs(fcs, &buf[..]));
71    }
72
73    #[test]
74    fn test_calculate_and_verify_fcs_uih() {
75        let buf = vec![
76            0b00010111, // Address Field - EA = 1, C/R = 1, DLCI = 5.
77            0b11101111, // Control Field - UIH command with P/F = 0.
78        ];
79
80        let fcs = calculate_fcs(&buf[..]);
81        assert!(verify_fcs(fcs, &buf[..]));
82    }
83
84    /// Tests calculating the FCS from the example frame provided in GSM 7.10 Annex B.1.
85    #[test]
86    fn test_calculate_fcs_sabm_example() {
87        // The expected FCS value is defined in GSM 7.10 Annex B.3.1.
88        let expected_fcs = 0b10001001;
89
90        // The frame is defined in GSM 7.10 Annex B.1.
91        let example_frame = [0b00000111, 0b00111111];
92
93        let fcs = calculate_fcs(&example_frame[..]);
94        assert_eq!(fcs, expected_fcs);
95    }
96}
bt_rfcomm/frame/fcs.rs

bt_rfcomm/frame/
fcs.rs
