题目给了一个txt文件，里面是swift的抽象语法树。
文本很长而且是我不熟悉的swift语言，所以先了解一下swift的抽象语法树吧。
swift开发环境搭建：https://www.fatbobman.com/posts/swift-in-linux/
例子：
var str = "hello"
使用如下命令将switf文件转换为ast文件。
swiftc -dump-ast hello.swift
如图就是一个初始化语句的ast结构：
◆pattern_binding_decl：初始化语句
◆pattern_named type=’String’ ‘str’：变量名称
◆value=”hello”：变量值
在题目给出的ast中，有类似的结构。
(pattern_binding_decl range=[re.swift:18:5 - line:18:15]
(pattern_named type='String' 'key')
Original init:
(string_literal_expr type='String' location=re.swift:18:15 range=[re.swift:18:15 - line:18:15] encoding=utf8 value="345y" builtin_initializer=Swift.(file).String extension.init(_builtinStringLiteral:utf8CodeUnitCount:isASCII:) initializer=**NULL**)
Processed init:
(string_literal_expr type='String' location=re.swift:18:15 range=[re.swift:18:15 - line:18:15] encoding=utf8 value="345y" builtin_initializer=Swift.(file).String extension.init(_builtinStringLiteral:utf8CodeUnitCount:isASCII:) initializer=**NULL**))
(var_decl range=[re.swift:18:9 - line:18:9] "key" type='String' interface type='String' access=fileprivate let readImpl=stored immutable)
所以这一块结构的意思就是：
var key = "345y"
再看一个例子：
var num = 10
if num == 10 {
num = 100
}
if语句
◆if_stmt：if语句
◆declref_expr：比较的对象
◆integer_literal_expr：比较的值
赋值语句
◆assign_expr：赋值语句
◆declref_expr：赋值的对象
◆integer_literal_expr：赋值的值
其他的对应关系就不列出了，可以自己写代码对照。
(call_expr type='()' location=re.swift:20:5 range=[re.swift:20:5 - line:20:17] nothrow
(declref_expr type='(Any..., String, String) -> ()' location=re.swift:20:5 range=[re.swift:20:5 - line:20:5] decl=Swift.(file).print(_:separator:terminator:) function_ref=single)
(argument_list labels=_:separator:terminator:
(argument
(vararg_expansion_expr implicit type='Any...' location=re.swift:20:11 range=[re.swift:20:11 - line:20:11]
(array_expr implicit type='Any...' location=re.swift:20:11 range=[re.swift:20:11 - line:20:11] initializer=**NULL**
(erasure_expr implicit type='Any' location=re.swift:20:11 range=[re.swift:20:11 - line:20:11]
(declref_expr type='Bool' location=re.swift:20:11 range=[re.swift:20:11 - line:20:11] decl=re.(file).top-level [email protected]:19:9 function_ref=unapplied)))))
(argument label=separator
(default_argument_expr implicit type='String' location=re.swift:20:10 range=[re.swift:20:10 - line:20:10] default_args_owner=Swift.(file).print(_:separator:terminator:) param=1))
(argument label=terminator
(default_argument_expr implicit type='String' location=re.swift:20:10 range=[re.swift:20:10 - line:20:10] default_args_owner=Swift.(file).print(_:separator:terminator:) param=2))
)))))))
◆call_expr：函数调用
◆declref_expr：调用的函数，decl=Swift.(file).print(_:separator:terminator:)，这里调用的是print函数
◆argument：参数
print(result)
这段代码中，调用了print函数，打印的是result的值，result是Bool类型的数据，所以程序会输出true或者false。
(pattern_binding_decl range=[re.swift:19:5 - line:19:33]
(pattern_named type='Bool' 'result')
Original init:
(call_expr type='Bool' location=re.swift:19:18 range=[re.swift:19:18 - line:19:33] nothrow
(declref_expr type='(String, String) -> Bool' location=re.swift:19:18 range=[re.swift:19:18 - line:19:18] decl=re.(file)[email protected]:1:6 function_ref=single)
(argument_list
(argument
(declref_expr type='String' location=re.swift:19:24 range=[re.swift:19:24 - line:19:24] decl=re.(file).top-level [email protected]:17:9 function_ref=unapplied))
(argument
(declref_expr type='String' location=re.swift:19:30 range=[re.swift:19:30 - line:19:30] decl=re.(file).top-level [email protected]:18:9 function_ref=unapplied))
))
Processed init:
(call_expr type='Bool' location=re.swift:19:18 range=[re.swift:19:18 - line:19:33] nothrow
(declref_expr type='(String, String) -> Bool' location=re.swift:19:18 range=[re.swift:19:18 - line:19:18] decl=re.(file)[email protected]:1:6 function_ref=single)
(argument_list
(argument
(declref_expr type='String' location=re.swift:19:24 range=[re.swift:19:24 - line:19:24] decl=re.(file).top-level [email protected]:17:9 function_ref=unapplied))
(argument
(declref_expr type='String' location=re.swift:19:30 range=[re.swift:19:30 - line:19:30] decl=re.(file).top-level [email protected]:18:9 function_ref=unapplied))
)))
(var_decl range=[re.swift:19:9 - line:19:9] "result" type='Bool' interface type='Bool' access=fileprivate let readImpl=stored immutable)
result变量就在call语句块的上方，可以分析出result是初始化的变量，用来初始化result变量的是一个call语句，调用的函数是check，参数分别是data和key。
var result = check(data, key)
(pattern_binding_decl range=[re.swift:18:5 - line:18:15]
(pattern_named type='String' 'key')
Original init:
(string_literal_expr type='String' location=re.swift:18:15 range=[re.swift:18:15 - line:18:15] encoding=utf8 value="345y" builtin_initializer=Swift.(file).String extension.init(_builtinStringLiteral:utf8CodeUnitCount:isASCII:) initializer=**NULL**)
Processed init:
(string_literal_expr type='String' location=re.swift:18:15 range=[re.swift:18:15 - line:18:15] encoding=utf8 value="345y" builtin_initializer=Swift.(file).String extension.init(_builtinStringLiteral:utf8CodeUnitCount:isASCII:) initializer=**NULL**))
(var_decl range=[re.swift:18:9 - line:18:9] "key" type='String' interface type='String' access=fileprivate let readImpl=stored immutable)
再往上可以找到key的定义。
var key = "345y"
(pattern_binding_decl range=[re.swift:17:5 - line:17:39]
(pattern_named type='String' 'data')
Original init:
(subscript_expr type='<null>'
(member_ref_expr type='@lvalue [String]' location=re.swift:17:28 range=[re.swift:17:16 - line:17:28] decl=Swift.(file).CommandLine.arguments
(type_expr type='CommandLine.Type' location=re.swift:17:16 range=[re.swift:17:16 - line:17:16] typerepr='CommandLine'))
(argument_list
(argument
(integer_literal_expr type='Int' location=re.swift:17:38 range=[re.swift:17:38 - line:17:38] value=1 builtin_initializer=Swift.(file).Int.init(_builtinIntegerLiteral:) initializer=**NULL**))
))
Processed init:
(load_expr implicit type='String' location=re.swift:17:37 range=[re.swift:17:16 - line:17:39]
(subscript_expr type='@lvalue String' location=re.swift:17:37 range=[re.swift:17:16 - line:17:39] decl=Swift.(file).Array extension.subscript(_:) [with (substitution_map generic_signature=<Element> (substitution Element -> String))]
(inout_expr implicit type='inout Array<String>' location=re.swift:17:16 range=[re.swift:17:16 - line:17:28]
(member_ref_expr type='@lvalue [String]' location=re.swift:17:28 range=[re.swift:17:16 - line:17:28] decl=Swift.(file).CommandLine.arguments
(type_expr type='CommandLine.Type' location=re.swift:17:16 range=[re.swift:17:16 - line:17:16] typerepr='CommandLine')))
(argument_list
(argument
(integer_literal_expr type='Int' location=re.swift:17:38 range=[re.swift:17:38 - line:17:38] value=1 builtin_initializer=Swift.(file).Int.init(_builtinIntegerLiteral:) initializer=**NULL**))
))))
(var_decl range=[re.swift:17:9 - line:17:9] "data" type='String' interface type='String' access=fileprivate let readImpl=stored immutable)
这里是data的定义，data是有一个subscript_expr语句块定义的，subscript_expr其实是在去数组的下标，这里取的是arguments[1]，也就是第二个命令行参数。
var data = CommandLine.arguments[1]
(if_stmt range=[re.swift:16:1 - line:21:1]
(binary_expr type='Bool' location=re.swift:16:32 range=[re.swift:16:4 - line:16:35] nothrow
(dot_syntax_call_expr implicit type='(Int, Int) -> Bool' location=re.swift:16:32 range=[re.swift:16:32 - line:16:32] nothrow
(declref_expr type='(Int.Type) -> (Int, Int) -> Bool' location=re.swift:16:32 range=[re.swift:16:32 - line:16:32] decl=Swift.(file).Int extension.>= function_ref=single)
(argument_list implicit
(argument
(type_expr implicit type='Int.Type' location=re.swift:16:32 range=[re.swift:16:32 - line:16:32] typerepr='Int'))
))
(argument_list implicit
(argument
(member_ref_expr type='Int' location=re.swift:16:26 range=[re.swift:16:4 - line:16:26] decl=Swift.(file).Array extension.count [with (substitution_map generic_signature=<Element> (substitution Element -> String))]
(load_expr implicit type='[String]' location=re.swift:16:16 range=[re.swift:16:4 - line:16:16]
(member_ref_expr type='@lvalue [String]' location=re.swift:16:16 range=[re.swift:16:4 - line:16:16] decl=Swift.(file).CommandLine.arguments
(type_expr type='CommandLine.Type' location=re.swift:16:4 range=[re.swift:16:4 - line:16:4] typerepr='CommandLine')))))
(argument
(integer_literal_expr type='Int' location=re.swift:16:35 range=[re.swift:16:35 - line:16:35] value=2 builtin_initializer=Swift.(file).Int.init(_builtinIntegerLiteral:) initializer=**NULL**))
))
在往上是一个if语句，操作符是 >=（extension.>=），操作数一个是argument.count()，一个是2,所以这一块的意思就是比较命令行参数是否大于等于2。
if CommandLine.arguments.count >= 2 {
}
在往上就是top_level_code_decl，顶层表达式了，如图所示，这个程序真是由一个func_decl和一个top_level_code_decl组成，我们已经分析完了下面的top_level_code_decl：
func check(encoded: String, keyValue:String) -> Bool {
}
if CommandLine.arguments.count >= 2 {
let data = CommandLine.arguments[1]
let key = "345y"
let result = check(encoded: data, keyValue: key)
print(result)
}
(func_decl range=[re.swift:1:1 - line:14:1] "check(_:_:)" interface type='(String, String) -> Bool' access=internal
(parameter_list range=[re.swift:1:11 - line:1:49]
(parameter "encoded" type='String' interface type='String')
(parameter "keyValue" type='String' interface type='String'))
(result
(type_ident
(component id='Bool' bind=Swift.(file).Bool)))
(brace_stmt range=[re.swift:1:59 - line:14:1]
上方函数的大体结构为：参数，返回值和一个表达式。
(parameter_list range=[re.swift:1:11 - line:1:49]
(parameter "encoded" type='String' interface type='String')
(parameter "keyValue" type='String' interface type='String'))
参数分别为encoded和keyValue，都是String类型。
(result
(type_ident
(component id='Bool' bind=Swift.(file).Bool)))
返回值为Bool类型。
(brace_stmt range=[re.swift:1:59 - line:14:1]
(pattern_binding_decl range=[re.swift:2:5 - line:2:33]
(var_decl range=[re.swift:2:9 - line:2:9] "b" type='[UInt8]' interface type='[UInt8]' access=private readImpl=stored writeImpl=stored readWriteImpl=stored)
(pattern_binding_decl range=[re.swift:3:5 - line:3:34]
(var_decl range=[re.swift:3:9 - line:3:9] "k" type='[UInt8]' interface type='[UInt8]' access=private readImpl=stored writeImpl=stored readWriteImpl=stored)
(pattern_binding_decl range=[re.swift:4:5 - line:4:25]
(var_decl range=[re.swift:4:9 - line:4:9] "r0" type='UInt8' interface type='UInt8' access=private readImpl=stored writeImpl=stored readWriteImpl=stored)
(var_decl range=[re.swift:4:13 - line:4:13] "r1" type='UInt8' interface type='UInt8' access=private readImpl=stored writeImpl=stored readWriteImpl=stored)
(var_decl range=[re.swift:4:17 - line:4:17] "r2" type='UInt8' interface type='UInt8' access=private readImpl=stored writeImpl=stored readWriteImpl=stored)
(var_decl range=[re.swift:4:21 - line:4:21] "r3" type='UInt8' interface type='UInt8' access=private readImpl=stored writeImpl=stored readWriteImpl=stored)
(for_each_stmt range=[re.swift:5:5 - line:12:5]
(return_stmt range=[re.swift:13:5 - line:13:198]
下方的语句块中又包含了3个定义变量的语句，1个for循环，1个返回语句。
(pattern_binding_decl range=[re.swift:2:5 - line:2:33]
(pattern_named type='[UInt8]' 'b')
Original init:
(call_expr type='[UInt8]' location=re.swift:2:19 range=[re.swift:2:13 - line:2:33] nothrow
(constructor_ref_call_expr type='(String.UTF8View) -> [UInt8]' location=re.swift:2:19 range=[re.swift:2:13 - line:2:19] nothrow
(declref_expr implicit type='(Array<UInt8>.Type) -> (String.UTF8View) -> Array<UInt8>' location=re.swift:2:19 range=[re.swift:2:19 - line:2:19] decl=Swift.(file).Array extension.init(_:) [with (substitution_map generic_signature=<Element, S where Element == S.Element, S : Sequence> (substitution Element -> UInt8) (substitution S -> String.UTF8View))] function_ref=single)
(argument_list implicit
(argument
(type_expr type='[UInt8].Type' location=re.swift:2:13 range=[re.swift:2:13 - line:2:19] typerepr='[UInt8]'))
))
(argument_list
(argument
(member_ref_expr type='String.UTF8View' location=re.swift:2:29 range=[re.swift:2:21 - line:2:29] decl=Swift.(file).String extension.utf8
(declref_expr type='String' location=re.swift:2:21 range=[re.swift:2:21 - line:2:21] decl=re.(file).check(_:_:)[email protected]:1:14 function_ref=unapplied)))
))
Processed init:
(call_expr type='[UInt8]' location=re.swift:2:19 range=[re.swift:2:13 - line:2:33] nothrow
第一个定义变量的语句，变量名为b，类型是一个[UInt8]数组，这个数组是由ebcoded（decl=re.(file).check(:🙂[email protected]）转换过来的。
var b = [UInt8](encoded.utf8)
(pattern_binding_decl range=[re.swift:3:5 - line:3:34]
(pattern_named type='[UInt8]' 'k')
Original init:
(call_expr type='[UInt8]' location=re.swift:3:19 range=[re.swift:3:13 - line:3:34] nothrow
(constructor_ref_call_expr type='(String.UTF8View) -> [UInt8]' location=re.swift:3:19 range=[re.swift:3:13 - line:3:19] nothrow
(declref_expr implicit type='(Array<UInt8>.Type) -> (String.UTF8View) -> Array<UInt8>' location=re.swift:3:19 range=[re.swift:3:19 - line:3:19] decl=Swift.(file).Array extension.init(_:) [with (substitution_map generic_signature=<Element, S where Element == S.Element, S : Sequence> (substitution Element -> UInt8) (substitution S -> String.UTF8View))] function_ref=single)
(argument_list implicit
(argument
(type_expr type='[UInt8].Type' location=re.swift:3:13 range=[re.swift:3:13 - line:3:19] typerepr='[UInt8]'))
))
(argument_list
(argument
(member_ref_expr type='String.UTF8View' location=re.swift:3:30 range=[re.swift:3:21 - line:3:30] decl=Swift.(file).String extension.utf8
(declref_expr type='String' location=re.swift:3:21 range=[re.swift:3:21 - line:3:21] decl=re.(file).check(_:_:)[email protected]:1:33 function_ref=unapplied)))
))
Processed init:
(call_expr type='[UInt8]' location=re.swift:3:19 range=[re.swift:3:13 - line:3:34] nothrow
同上，接着定义一个变量k。
var k = [UInt8](keyValue.utf8)
(pattern_binding_decl range=[re.swift:4:5 - line:4:25]
(pattern_typed type='UInt8'
(pattern_named type='UInt8' 'r0')
(type_ident
(component id='UInt8' bind=Swift.(file).UInt8)))
(pattern_typed type='UInt8'
(pattern_named type='UInt8' 'r1')
(type_ident
(component id='UInt8' bind=Swift.(file).UInt8)))
(pattern_typed type='UInt8'
(pattern_named type='UInt8' 'r2')
(type_ident
(component id='UInt8' bind=Swift.(file).UInt8)))
(pattern_typed type='UInt8'
(pattern_named type='UInt8' 'r3')
(type_ident
(component id='UInt8' bind=Swift.(file).UInt8))))
(var_decl range=[re.swift:4:9 - line:4:9] "r0" type='UInt8' interface type='UInt8' access=private readImpl=stored writeImpl=stored readWriteImpl=stored)
(var_decl range=[re.swift:4:13 - line:4:13] "r1" type='UInt8' interface type='UInt8' access=private readImpl=stored writeImpl=stored readWriteImpl=stored)
(var_decl range=[re.swift:4:17 - line:4:17] "r2" type='UInt8' interface type='UInt8' access=private readImpl=stored writeImpl=stored readWriteImpl=stored)
(var_decl range=[re.swift:4:21 - line:4:21] "r3" type='UInt8' interface type='UInt8' access=private readImpl=stored writeImpl=stored readWriteImpl=stored)
接下来的语句块中定义了四个未初始化的变量r0, r1, r2, r3，类型是UInt8。
var r0, r1, r2, r3:UInt8
(dot_syntax_call_expr implicit type='(Int, Int) -> ClosedRange<Int>' location=re.swift:5:15 range=[re.swift:5:15 - line:5:15] nothrow
(declref_expr type='(Int.Type) -> (Int, Int) -> ClosedRange<Int>' location=re.swift:5:15 range=[re.swift:5:15 - line:5:15] decl=Swift.(file).Comparable extension.... [with (substitution_map generic_signature=<Self where Self : Comparable> (substitution Self -> Int))] function_ref=double)
(argument_list implicit
(argument
(type_expr implicit type='Int.Type' location=re.swift:5:15 range=[re.swift:5:15 - line:5:15] typerepr='Int'))
))
(argument_list implicit
可以看到(decl=Swift.(file).Comparable extension….),说明这是一个for-in循环。
(argument
(integer_literal_expr type='Int' location=re.swift:5:14 range=[re.swift:5:14 - line:5:14] value=0 builtin_initializer=Swift.(file).Int.init(_builtinIntegerLiteral:) initializer=**NULL**))
(argument
(binary_expr type='Int' location=re.swift:5:25 range=[re.swift:5:18 - line:5:26] nothrow
(dot_syntax_call_expr implicit type='(Int, Int) -> Int' location=re.swift:5:25 range=[re.swift:5:25 - line:5:25] nothrow
(declref_expr type='(Int.Type) -> (Int, Int) -> Int' location=re.swift:5:25 range=[re.swift:5:25 - line:5:25] decl=Swift.(file).Int extension.- function_ref=double)
(argument_list implicit
(argument
(type_expr implicit type='Int.Type' location=re.swift:5:25 range=[re.swift:5:25 - line:5:25] typerepr='Int'))
))
(argument_list implicit
(argument
(member_ref_expr type='Int' location=re.swift:5:20 range=[re.swift:5:18 - line:5:20] decl=Swift.(file).Array extension.count [with (substitution_map generic_signature=<Element> (substitution Element -> UInt8))]
(load_expr implicit type='[UInt8]' location=re.swift:5:18 range=[re.swift:5:18 - line:5:18]
(declref_expr type='@lvalue [UInt8]' location=re.swift:5:18 range=[re.swift:5:18 - line:5:18] decl=re.(file).check(_:_:)[email protected]:2:9 function_ref=unapplied))))
(argument
(integer_literal_expr type='Int' location=re.swift:5:26 range=[re.swift:5:26 - line:5:26] value=4 builtin_initializer=Swift.(file).Int.init(_builtinIntegerLiteral:) initializer=**NULL**))
)))
循环的范围是从0到b.count-4。
for i in 0...b.count-4 {
}
紧跟着的是六条赋值语句。
展开发现首尾的赋值语句格式相似,中间四条赋值语句格式相似,所以我们应该只需要仔细分析两种不同格式中的各一种。
(load_expr implicit type='UInt8' location=re.swift:6:30 range=[re.swift:6:29 - line:6:32]
(subscript_expr type='@lvalue UInt8' location=re.swift:6:30 range=[re.swift:6:29 - line:6:32] decl=Swift.(file).Array extension.subscript(_:) [with (substitution_map generic_signature=<Element> (substitution Element -> UInt8))]
(inout_expr implicit type='inout Array<UInt8>' location=re.swift:6:29 range=[re.swift:6:29 - line:6:29]
(declref_expr type='@lvalue [UInt8]' location=re.swift:6:29 range=[re.swift:6:29 - line:6:29] decl=re.(file).check(_:_:)[email protected]:2:9 function_ref=unapplied))
(argument_list
(argument
(declref_expr type='Int' location=re.swift:6:31 range=[re.swift:6:31 - line:6:31] decl=re.(file).check(_:_:)[email protected]:5:9 function_ref=unapplied))
)))
这是第一个赋值语句的结构,做的事情是用b数组的值填充r0到r3。
r0, r1, r2, r3 = b[i], b[i+1], b[i+2], b[i+3]
第二个赋值语句,首先是一个取下标,取的是b[i+0]。
接着是一个运算语句块。
初步看出是用r2异或一个表达式。
该表达式是另一个表达式 & 0xff得到。
接着又是加运算。
一个是k[0]。
一个是r0 >> 4。
综上,第二个赋值语句:
b[i+0] = r2 ^ ((k[0] + (r0 >> 4)) & 0xff)
同理可以分析出接下来的几条赋值语句。
b[i+1] = r3 ^ ((k[1] + (r1 >> 2)) & 0xff)
b[i+2] = r0 ^ k[2]
b[i+3] = r1 ^ k[3]
k[0], k[1], k[2], k[3] = k[1], k[2], k[3], k[0]
for循环之后就是返回语句。
返回语句由一条判断语句组成，操作符是==。
第一个参数是b数组。
第二个参数是一个给定的数组。
[88, 35, 88, 225, 7, 201, 57, 94, 77, 56, 75, 168, 72, 218, 64, 91, 16, 101, 32, 207, 73, 130, 74, 128, 76, 201, 16, 248, 41, 205, 103, 84, 91, 99, 79, 202, 22, 131, 63, 255, 20, 16]
return (b == [88, 35, 88, 225, 7, 201, 57, 94, 77, 56, 75, 168, 72, 218, 64, 91, 16, 101, 32, 207, 73, 130, 74, 128, 76, 201, 16, 248, 41, 205, 103, 84, 91, 99, 79, 202, 22, 131, 63, 255, 20, 16])
re.swift
func check(encoded: String, keyValue:String) -> Bool {
var b = [UInt8](encoded.utf8)
var k = [UInt8](keyValue.utf8)
var r0, r1, r2, r3:UInt8
for i in 0...b.count-4 {
r0 = b[i]
r1 = b[i+1]
r2 = b[i+2]
r3 = b[i+3]
b[i+0] = r2 ^ ((k[0] + (r0 >> 4)) & 0xff)
b[i+1] = r3 ^ ((k[1] + (r1 >> 2)) & 0xff)
b[i+2] = r0 ^ k[2]
b[i+3] = r1 ^ k[3]
let temp = k[0]
k[0] = k[1]
k[1] = k[2]
k[2] = k[3]
k[3] = temp
}
return (b == [88, 35, 88, 225, 7, 201, 57, 94, 77, 56, 75, 168, 72, 218, 64, 91, 16, 101, 32, 207, 73, 130, 74, 128, 76, 201, 16, 248, 41, 205, 103, 84, 91, 99, 79, 202, 22, 131, 63, 255, 20, 16])
}
if CommandLine.arguments.count >= 2 {
let data = CommandLine.arguments[1]
let key = "345y"
let result = check(encoded: data, keyValue: key)
print(result)
}
解题脚本
b = [88, 35, 88, 225, 7, 201, 57, 94, 77, 56, 75, 168, 72, 218, 64, 91, 16, 101, 32, 207, 73, 130, 74, 128, 76, 201, 16, 248, 41, 205, 103, 84, 91, 99, 79, 202, 22, 131, 63, 255, 20, 16]
k = [121, 51, 52, 53] //换位后的key
for i in range(len(b)-4, -1, -1):
k[1], k[2], k[3], k[0] = k[0], k[1], k[2], k[3]
r1 = k[3] ^ b[i+3]
r0 = k[2] ^ b[i+2]
r3 = ((k[1] + (r1 >> 2)) & 0xff) ^ b[i+1]
r2 = ((k[0] + (r0 >> 4)) & 0xff) ^ b[i]
b[i], b[i+1], b[i+2], b[i+3] = r0, r1, r2, r3
for i in b:
print(chr(i), end='')
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原文始发于微信公众号（看雪学苑）：国赛babytree 赛题解析
