Add asn1grep tool

Scrolling through the ASN.1 files for datatypes is a nightmare, so I
wrote a script to automate that

tools/README.md

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+## Rayhunter tools
+
+### `asn1grep.py`: a script for finding a datatype in ASN.1 files
+
+`asn1grep` parses our ASN.1 spec files, then searches for a given datatype by recursively descending through the LTE-RRC types we care about. it then prints out each result as a "path" through the highly nested datatypes.
+
+Setup:
+1. `python -m venv .venv && . .venv/bin/activate`
+2. `pip install -r requirements.txt`
+
+Usage:
+```
+» python asn1grep.py IMSI                                  
+searching for IMSI
+PCCH-Message [message [message.c1 [c1 [c1.paging [paging [pagingRecordList[0] [ [ue-Identity [ue-Identity.imsi [IMSI]]]]]]]]]]
+```

tools/asn1grep.py

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+import asn1tools
+import sys
+
+ASN_FILES = [
+    '../telcom-parser/specs/PC5-RRC-Definitions.asn',
+    '../telcom-parser/specs/EUTRA-RRC-Definitions.asn',
+]
+
+TERMINATING_TYPE_NAMES = [
+    'DL-CCCH-Message',
+    'DL-DCCH-Message',
+    'UL-CCCH-Message',
+    'UL-DCCH-Message',
+    'BCCH-BCH-Message',
+    'BCCH-DL-SCH-Message',
+    'PCCH-Message',
+    'MCCH-Message',
+    'SC-MCCH-Message-r13',
+    'BCCH-BCH-Message-MBMS',
+    'BCCH-DL-SCH-Message-BR',
+    'BCCH-DL-SCH-Message-MBMS',
+    'SBCCH-SL-BCH-Message',
+    'SBCCH-SL-BCH-Message-V2X-r14',
+]
+
+def load_asn():
+    return asn1tools.compile_files(ASN_FILES, cache_dir=".cache")
+
+def get_terminating_types(rrc_asn):
+    return [rrc_asn.types[name] for name in TERMINATING_TYPE_NAMES]
+
+def search_type(haystack, needle):
+    if haystack.type_name == needle or haystack.name == needle:
+        return [needle]
+
+    result = []
+    if 'members' in haystack.__dict__:
+        for name, member in haystack.name_to_member.items():
+            for member_result in search_type(member, needle):
+                result.append(f"{haystack.name}.{name} [{member_result}]")
+    elif 'root_members' in haystack.__dict__:
+        for member in haystack.root_members:
+            for member_result in search_type(member, needle):
+                result.append(f"{haystack.name} [{member_result}]")
+    elif 'element_type' in haystack.__dict__:
+        for element_result in search_type(haystack.element_type, needle):
+            result.append(f"{haystack.name}[0] [{element_result}]")
+    elif 'inner' in haystack.__dict__:
+        for inner_result in search_type(haystack.inner, needle):
+            result.append(f"{haystack.name} [{inner_result}]")
+
+    return result
+
+
+if __name__ == "__main__":
+    type_name = sys.argv[1]
+    print(f"searching for {type_name}")
+
+    rrc_asn = load_asn()
+    terminating_types = get_terminating_types(rrc_asn)
+    needle = rrc_asn.types.get(type_name)
+    if needle == None:
+        raise ValueError(f"couldn't find type {type}")
+
+    for haystack in terminating_types:
+        for result in search_type(haystack.type, type_name):
+            print(result)

tools/requirements.txt

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+asn1tools==0.166.0
+bitstruct==8.19.0
+diskcache==5.6.3
+pyparsing==3.1.2